Hi Hudson,
We tend to view takeoff performance i.e. Assumed Temp and V Speeds, as one fixed set of numbers for a particular TOW. This is only true if you are taking off at the max. limited TOW on a balanced field.
At any weight that is lower than the max. limited TOW, we are opening up a range of Ass. Temp as well as V speeds. This fact will be clearer if we were to calculate our takeoff performance using the Generalised Method which is similar to Performance A exam.
But in the real world (as in my company), we have Specific Takeoff tables to help simplify our task. Instead of having a range of speeds, we get a fixed set of speeds together with a fixed Ass. temp. Unfortunately, in simplifying things, the workings behind it is discarded and over time it can become confusing (i know it did for me).
If you read carefully, Mutt, John and Alatriste have pretty much answer your question.
PS: Tuur, Alatriste is right, the V speeds does change with thrust. In fact, V speeds INCREASE with INCREASE Ass. Temp (REDUCING thrust). This is to use the increased V speed (utilising the extra runway length) to counter the decreased in second segment climb performance.