The situation with the speed of old "78 rpm" shellac discs is much simpler than most people today think it is.
When shellac discs were first introduced in the late 19th century different speeds were used by different record companies ranging from 60–130 rpm. Sizes of the disc also varied. Around the turn of the century most companies used a speed of about 76–80 rpm. Grammophones back then came with speed regulators that allowed you to manually adjust the speed to the rpm used by any given record company. In 1925 the speed became standardised at 78 rpm and the most popular size was the 10" disc (which holds about 3 mins of music at 78 rpm). So people who grew up in the shellac disc era were used to records having been recorded/mastered at different speeds. It was no big deal, they simply adjusted the playback speed of their grammophones based on experience or information provided by the record companies or until it sounded right to their ears.

So speeds on old shellac discs differ for a variety of reasons:

a) company practice/policy
b) non-standardized, sometimes unreliable early recording technology
c) songs that ran over 3 mins sometimes were sped up to fit on 10" discs
d) songs were sped up to make them more exciting

But again, this was no big deal back then. People simply adjusted the playback speed of their grammophones. It was a standard practice in those days. Rpm was a flexible thing in the pre-HiFi-world. The concept of truly and mechanically fixed speeds of 45 rpm and 33 1/3 rpm was unknown in the shellac disc era.

Playback speed of old shellac discs only became a problem when people no longer played the original discs on the original equipment e.g. when shellac discs were re-released on LPs and later CDs. In most cases these re-releases were made by playing and then transferring the old discs to tape (and later digital tape or hard drives). So what you hear on CDs today depends on how the old discs were played back by the people who transferred them to tape or digital media. MOST old shellac discs were played back and transferred at the wrong speed simply because "78 rpm" today is considered a fixed and true playback speed, when in actual fact – as pointed out above – each disc has to be adjusted individually.

Not only do Robert Johnson's discs play at the wrong speed on today's CDs, the speed also varies from track to track. Some seem to be correct, some are obviously too fast, some are somewhere in between. It is not as simple as "20% too fast" as some have argued. And 20% is too much anyway. I'd say speeds on the MOST RECENT and OFFICIAL Johnson CDs ("King Of The Delta Blues Singers Vol. 1 & 2", Sony, 1998/2004) are everywhere from correct to 5 maybe 10% too fast.
It's not as if Sony are not aware of the problem: the 1990 "Complete Robert Johnson" (2 CDs) was mastered from a 1974 compilation tape of inferior quality and was quietly replaced by Sony in 1996 by a remastered/corrected "Complete Robert Johnson" (the 1990 and 1996 sets look identical, you have to check the production date). The problem was further adressed by Sony (again without much fanfare) on the 1998 and 2004 single disc sets which apparantly were taken directly off the original discs. Still playback speeds vary.

Robert Johnson grew up in the shellac disc era. He was one of the first generation of blues players who were influenced by listening to records AND by listening to live music. Many melodies, riffs and lyrics on Johnson's records can be traced back to specific discs that Johnson must have owned/known. So as a record collector/listener Johnson was familiar with the standard practice of his time namely to manually adjust the playback speed of any given disc. This answers the question "Why didn't Johnson notice that his records were playing too fast?" They did not play too fast back then, because playback speed was adjusted. It's just that in today's world of rigidly fixed playback speeds this once common practice is no longer known. Johnson did not listen to the sound of his voice on the CDs we have today. He listened to original records. On original playback equipment, which makes world of difference.