Score is stored in most system 11 games as 4 bytes so [12][34][56][78] for the digits. I think Whirlwind handles this correctly as someone got 122 million on mine a long time ago (and only stopped because the right flipper link melted) and it displayed correctly but this was 10+ years ago so don't remember for sure.

Some games recognize the overflow so you can get credit for the score but it won't be able to store it correctly. It's not so much a matter of space but there's a lot of extra programming involved (although not too much if your core routines are set up correctly) to add digits. Not just the actual addition of score, but audit routines, animation routines, flat out space to show the digits (they already bumper up 10 millions scores P1-P2 which can be hard to discern), etc.

You can get /10 roms that could "fix" this at least untill you break 999 million! They are definitely a hack though.

It's not limited to 11c BTW, it's really just a decision by the game programmer how they handled this. F-14 recognizes >10 scores up to 20 million, and sets the HSTD to all 9's. (Black Knight does this too, way back in the early days of that programming system which spans system 7 to 11c). I forget what Pinbot does but it's really an issue on that game since its stock form doesn't cap the solar value like it should and you can turn it over easily.