AD update synch always showing large numbers of users need to synch

When we are doing an update AD synch for Servicedesk the status nearly always says over 20% of the users need synching, which is puzzling.

There should be only a few that need synching and updating - not 20% of the user base. Some of these will probably be accounts where that use the same email address as other accounts but the % remains too high to make any sense.

Any thoughts as to why this should be?

Cheers

Steve