I thought I understood this, but maybe I don't. What is the meaning of a polyphony number over 88?

Seems strange on a piano, doesn't it? Actually it makes sense if you consider that if you record a multi-track song with multiple instruments you could exceed 88. Imagine recording a song with piano, bass, guitar, drums, strings, cowbells, sitar, and etc... all playing on a single keyboard.

There have been many threads on this forum explaining polyphony. Use the "search" function and you'll turn up dozens. For example,
http://www.pianoworld.com/ubb/cgi-bin/ultimatebb.cgi?/topic/6/351.html

Classic issue for digital pianos. No one really know actual keys available at one time except original manufacturer. Why?
Digital pianos nowadays often use more than one voice at any given key strike. How many numbers? It depends on velocity of keystrike and the way it was programmed by manufacturer. At one point, a key hit may use 2 voices while a harder key hit may up to 6 voices so it sounds real loud and big(to emulate real piano). That is why people prefer more than 88 voices polyphony to avoid notes drop out or premature cut off of sustained notes. Funny thing is keys you hold with your fingers never drop out no matter what.. ONly notes on sustain drop out. I don't know if I have more than 10 finger..what would happen?