You've been lucky. Like I said, it depends on the RAM used, and the SPD information included on that RAM, along with the specific model of Mac. Since you're talking about Macs that old, a lot of PC133 RAM does not work properly in Cubes for example.
Even worse, in one example, off-spec but faster RAM that did work in a specific model of Mac stopped working after a firmware update, because Apple tightened up the spec requirements.
Thus, as a rule of thumb, I ALWAYS tell people to buy properly spec'd RAM for Macs. You're asking for trouble if you use better spec'd but off-spec RAM. BTW, in modern times, this is true even for the 2017 27" iMac. Some off-spec faster RAM just refused to boot, whereas if you bought properly spec'd cheaper RAM, it worked fine. There's nothing inherently wrong with the faster RAM and it would run perfectly fine at the slower speeds, but some third party RAM manufacturers will not include the proper SPD info for slower speeds in that RAM, and in that scenario, sometimes the Mac will not boot properly because of this.
pretty much every mac under the Sun can work with RAM which is rated for faster speeds then what the Mac runs its RAM at.
pretty much all RAM since SDRAM (and even some EDO RAM) use SPD JDEC Timings in that theres a small EEPROM that contains infomation on the RAM stick at hand
and there will be a few settings for the various speeds the RAM sticks support
for example a 1600Mhz RAM stick will also almost always contain information for if a Computer wants to run the RAM at say 1066Mhz and 1333Mhz
so if I install say a 1600Mhz RAM stick into my MacPro5,1 it will simply "go" "Ok I only support up to 1333Mhz RAM let me see what the 1333Mhz SPD table says for this RAM stick"
(hell in the case of the G4 cube it dosent even support any sort of Memory dividers at all, the RAM will always run at 100Mhz synchronous with the 100Mhz CPU bus so 133Mhz RAM will run just fine as the G4 cube will just ignore the 133Mhz SPD Timing table and just run the RAM at 100Mhz regardless)
but the exception is MCP79/MCP89 macs as they are dumb in that, if you install a RAM stick faster then what they are specced for instead of the Mac going "Ok whats the fastest I actually support" and looking for that table in SPD, they just go straight for whatever is the highest speed and they try and run at that speed, which they obviously dont support so you get the 3 Beeps of No Good Banks
sometimes the machine will actually POST at the higher speeds, but can end up being very unstable as you have effectively overclocked the Memory controller,
@MagicBoy experienced with his 2009 MCP79 Mac Mini IIRC
(and very rarely you will actually get a machine that willy happily hum along at 1333Mhz, most of these cases have been on the newer MCP89 macs)
