Millerina,
Clare's suggestion is the most likely explanation.
There is yet another possibility for the age discrepancy - it's rather complicated so I mention it only because I've come across exactly this situation.
A (remote?) possibility is that William was the issue of a previous marriage and that his birth mother died and his father remarried to Janet MCIVOR; but William always thought that Janet (MCIVOR/MCKIVER) POLLOCK was actually his mother because he'd only been 2 or 3 when she came into his life and so he remembered no other mother ...
In the other case, as I dimly recall (might not be exact), the father came from Ireland, the son had been born in Ireland, and the father had re-married in Scotland to a Scottish girl when his son was very small.
I wonder what other information you have about William, and about Thomas and Janet. As you probably know, for deaths on SP a married woman is recorded under both her married name and (if known) her maiden name. So you could search for the death of a Janet POLLOCK (I guess there'll be quite a number!) and for the death of a Janet MCIVOR (or, perhaps, M*C*V*R). If you get a death in each search with the same date and reference number, the resultant certificate might give you some clues which you can compare with other information which you have.
Regards,
JAP