Hi Kiran,
My few cents, as Raghuraman said if you are in PI 7.4 SP11, before a new SP, SAP customers or SAP itself detect urgent issues that they can't wait to a new SP and therefore you can have several patch level, PL 0001, PL 0002 and so on.
When a new SP is released can have the corrections of previous PL SP's but sometimes not, even you can see corrections thorough several PI versions.
Summing up, you can have:
PI 7.31 SP 000 PL 000
PI 7.31 SP 001 PL 000
PI 7.31 SP 001 PL 001
PI 7.31 SP 001 PL 002
PI 7.4 SP 000 PL 000
PI 7.4 SP 000 PL 001
and so on.
Regards.