every transmission fluid expands when hot - infact even engine oil and radiator coolant also expand when hot.
If you notice on a transmission dipstick it has a hot and cold mark - the reason the hot one is above the cold one is to measure it on both temperatures.
These "overflow" type transmissions require a very specific temperature because that was the easiest for a flushing and filling. However toyota do make a tool that can measure the fluid level at full operating temperature. But it also requires the use of a vacuum pump so the oil doesnt pour out when you remove the drain bolt as the hot level will be higher than it.