Paul,

Because "the meditations" Is a thesis on the mind body problem, which is the subject being discussed.

"A discourse on Method" is a thesis on scientfic method and therefore the quotes you refer to are in an entirely different context.

So if you want to know what Descartes analysis of the mind body problem is, you need to refer to the world which covers that subject.

As Crow Sister points out, he modified his language when he wrote the meditations to more accurately reflect what he had discovered, as he wanted to clarify that he was not merely assured of his own existance due to some logical word trick, but because he had applied his method of close and careful observation and scrutiny and found that there were no circumstances under which he could make any other claim.

In other words, no matter how much he tried to doubt "I", it always went and hid round the back of a verb ...

I doubt "I" ... oops - "I" just doubted something - empirically I have to admit that.

It is confusing yes, but nonetheless correct.