I have an old design, since 1999 approximately, with an old FPGA XCS30XL and old MCU6840 MCU. They are connected together via a parallel bus peripheral and both utilise and power off 5V. But there is an oddity which is that some of the control lines between the two (IRQs, CSs, R,W,etc) are pulled to 3.3V instead of 5V (Address/data are a mix pulled to 5V or no pullups). All other pullups in the system are generally 5V,
What could the reason/benefits be for pulling a 5V signaling system (at 16Mhz or so) to 3.3V instead of 5V in push/pull logic when the chips actually use 5V signaling?