They changed all Steam IDs from STEAM_0: to STEAM_1
"Some peoples IDs aren't 1, STEAM_2:0:, STEAM_1:1:, etc. but using the steam_1 method will fix most, if you want an even easier way just remove the STEAM_0:0: part from every ID in the database and only use the unique numbers at the end. As an example my STEAMID was STEAM_0:0:13214556, it is now STEAM_1:0:13214556, removing the STEAM_0:0: part would leave you with 13214556 which is still a unique number. I believe this is the method the statistics system uses to" - Some Guy Named WildFire
TF2 and DoD:S are using the new STEAM_1 ID and CS:S is using the old STEAM_0 ID. This means each of the 600+ people has to have a SECOND ID added for each person (a STEAM_1 and a STEAM_0 ID).
It probably has to do with Steam Cloud.
L4D uses Steam Cloud and has STEAM_1 SteamIDs
TF2 now uses Steam Cloud and has STEAM_1
STEAM_1:0:1337 and STEAM_0:0:1337 are the same.
STEAM_0:1:1337 and STEAM_0:0:1337 are different steam accounts.
I saw this on the Steampowered forums.