ajandjjmum It's about vote distribution.
Imagine the voting population was 40 people, that there were only 2 parties, only 8 seats, and each seat had only 5 voters, and both the Labour and Tories got 20 votes in total each.
If you put Labour votes first, and Tory votes second, you could imagine a vote distribution like
Seat 1: 3 2
Seat 2: 3 2
Seat 3: 3 2
Seat 4: 3 2
Seat 5: 2 3
Seat 6: 2 3
Seat 7: 2 3
Seat 8: 2 3
In that case both Labour and Tories would get 4 seats each.
But you could also have a distribution like
Seat 1: 3 2
Seat 2: 3 2
Seat 3: 3 2
Seat 4: 3 2
Seat 5: 3 2
Seat 6: 3 2
Seat 7: 1 4
Seat 8: 1 4
in which case Labour would have 6 seats, and the Tories 2, even though both still get 20 votes in total.
This is because the Tories are piling up 'extra' votes in seats 7 and 8, where they don't need them. In theory, winning by more than 1 vote is wasteful, in the sense the other votes could be better used elsewhere. And getting any votes in a seat you lose is wasteful, because those votes would be better in a marginal. Of course, in practice, MPs don't like winning by 1 vote very much...
Labour's vote is traditionally more efficient than the Tories, in a way similar to this example. I think this advantage will be slightly reduced this time round, partly because many Labour votes in Scotland will be wasted - and because UKIP will take some useless votes in very safe Tory seats.
In recent history, the biggest losers from this wasted votes syndrome were the Lib Dems, but this time round, first-past-the-post will ironically save them many seats, because they will only really get any votes where they have a sitting MP. Their vote distribution is likely to be very efficient in 2015.
On top of the above wasted votes syndrome, there is a second element in the current voting system which works against the Tories. The sizes of constituencies are not all equal. If all the constituencies had an equal number of eligible voters, the Tories would gain about 20 seats. But this legislation wasn't enacted, despite being in the coalition agreement. You can take your pick as to whether it was betrayal by the LDs, who got their AV referendum to which it was tied in the coalition agreement, or principle by the LDs, who did not get their House of Lords reform, though it was not linked in the coalition agreement, or self-interest by the LDs, who would have lost seats to the Tories if it were enacted. I personally regard it as a mix of the three, but mainly betrayal and self-interest.