The concatenation adds together Timer2 + Timer2 Prescaler + internal clock divider. So, if the prescaller was set to 32 then the following would be incremented.
First the 2 internal clock divider bits are incremented. (2 bits)
If there is an overflow from above then the prescaler is incremented. (5 bits)
If there is an overflow from the prescaler then Timer2 is incremented. (8 bits)
The top 10 bits are used in the comparison. In this case Timer2 + the two most significant bits of the prescaler.
If the prescaller is set to 2 which is only 1 bit then the 10 bits are Timer2 (8 bits) + Prescaler (1 bit) + Internal Divider(1 bit).
Mike.