You are correct, that 115v transformers on 120v lines will make more voltage. And VoltSecond is correct (of course!), that if you do not use the full rated current of the transformer, the voltage will be higher still. The latter is pretty common, since most DIYers think that a transformer with excess current capability will run cooler (it will) and that is better (not always!).
Bottlehead power transformers are designed for 120v nominal input. Many of them have unused current capacity, but are designed to give the right voltages in the circuits we use. We can di this because they are built for the specific circuit.
VoltSecond also makes the point about magnetic headroom. Running a transformer at maximum flux density makes it possible to use a smaller transformer, with less iron and less copper - so it's cheaper. Many standard transformers run 15kGauss to 16.5kGauss, which is the nominal maximum for typical grain-oriented silicon steel. I try to stay under 10kGauss in our units, so they will operate properly even at 50Hz and even at 130v - a +/-10% variation in line voltage is not uncommon these days.