Some Macs have hardware memory controller limits - only so many address lines. Many are limited by the boot ROM to a certain ceiling. I think the Classic II is one of these. There are some which will take more memory than Apple states, and those are found by somebody experimenting, putting in bigger compatible chips to see what happens. Usually, the extra memory is ignored. Sometimes the system doesn't boot, sometimes the refresh cycle is wrong and the memory is flaky, and occasionally the supply Voltage is wrong and the memory is destroyed (not a risk with 30-pin RAM, which is all 5V). The Mac II is limited to 256K & 1MB SIMMs by the video circuit, which accidentally puts 4MB & 16 MB SIMMs into factory test mode. There were special 4MB "PAL" SIMMs which were Mac II compatible, but they are very rare.