Anglo - that doesn't answer the question asked, which was that the questioner has seen references using two different spellings of a name, and wants to know if they are both about the same person (one spelling incorrect), or about two totally different people with a similar name.
Let's face it. There are a lot of people out there, and there could be hundreds of people out there with both spellings of a name (no matter how obscure). Any one of them could be on the web for different or similar reasons.
I don't know the name, but would not just rely on ONE reference found in Wikipedia to prove that some other person with a similar name does NOT exist.

Weasel - perhaps it would be helpful if you gave links to the different referencs using a different spelling. That way, people who know something about "Alasdair Roberts" may be able to identify if the other "Alaisdair Roberts" is the same person or otherwise.
Quack!
Geoff the Duck.