1480 is just default safe value. If you are absolutely sure that 1500 byte packets are forwarded without any problems then you can easily set pppoe mtu to 1492
Alright, I have done so and have had good luck so far.
What about the connections that dont take the default value, what causes this to happen?
Example:
I connected a device that was hard set to 1492, mikrotik was set to 1492 and the pppoe-interface on the mikrotik shows 1488 for that connection.