so what's the difference between the round length and a block? I assumed that the round was just as long as it took to find a block?
The round is the time it takes somebody in the pool to solve a block, not for the block itself to be mined. During this time, many other blocks may have been solved by other pools or individual miners. If you look at the statistics page you will see the round number in the first column - these are consecutive, and relate only to Slush's pool. Further across is the block number. Compare this to the previous entry, and you will see that the number has probably increased by more than one. This is because the intermediate blocks went elsewhere.
Okay, specific recent example:
21138 2013-12-12 12:17:43 0:10:04 66923249 1134 0.00041559 274494 25.05831750 94 confirmations left
21137 2013-12-12 12:07:39 0:35:35 237881115 4273 0.00047107 274491 25.11792581 91 confirmations left
Round 21138 took Slush's pool 10'04" to complete since round 21137, which solved block number 274491, so the two intermediate blocks - 274492 and 274493 - were mined by somebody else. We solved 274494 at 12:17:43, the block itself having taken just under eight minutes from when the previous block (274493) was solved at 12:09:58 elsewhere.
Rounds therefore do not equate to blocks, except when our pool happens to solve consecutive blocks in quick succession.