What's the advantage of using non-truncated mining targets?
You are right, it could happen you are finding a hash that meets the pool target but not the highest possible target. But because the difficulty is way higher than one, and therefore the share wouldnt finish the block anyway, its not in their interest. You have done your work and you will get your share. They just make it a bit easier for you, by passing a higher target than possible.

Another possible reason:
Comparing a hash to the highest possible target would take more time, because you have to check the whole hash (checking the zeros at the end as well). With the non-truncated target you only need to check until the first F occures because than its at least equal or lower.