Youe Clock_2 is running at 12MHz and not at 10 as your calculations show. Furthermore is the precision of your clocks 2%, not 1%.
Ok, but the clock_2 isn't related with the counting precision. The precision is due by the clock_1.
I've checked the internal clock (Pin_1 and PWM_Out) and they seem to be precise even better than 2%, and LCD readout is more imprecise.
Anyway, I'll try with an external clock.
Thanks for your help.