Arduino Rounding question
This isn't down to rounding, but down to the way the characters are displayed on your lcd. Once you display 100 you are displaying three characters on your display then when it drops down to 99 you are only displaying 2 characters, yet the display shunts it right leaving the 0 from the 100 still displayed. 

Not sure of the exact solution but try experimenting with the lcd.setCursor(9,1); and lcd.setCursor(9,0); functions.

Or call the lcd.clear(); function somewhere in your void loop();