The latest release of SQL 2012 updates has risen a little bit of confusion, especially around why have we released SP3 CU1 before SP2 CU10 meaning SP3 CU1 doesn’t contain all the previous fixes (SP2 CU10s fixes).
The short answer to this is that each service pack creates a baseline for customers and has its own support lifecycle. This means that customers who decide not to upgrade to SP3 straight away will still be getting SP2 CUs until it goes out of mainstream support.
We do have a process of forward porting all fixes to the next release so in this instance SP3 CU2 will contain the fixes from SP2 CU10.
For more information, check out the Incremental Servicing Model which details the update lifecycle - https://support.microsoft.com/en-us/kb/935897