Quite correct JCB,
the call is so that the ATC can verify your mode C readout. The requirement is to report 'to the nearest 100 feet'.
Even though, for example, FL107 would not be allocated as cruise level, you should report FL107 as the level passing. This is different to reporting 'last vacated level' as in step climb/descents.
Of course, the difference in report altitude as feet eg. 9,600' and as a flight level is simply that you are referencing Std 1013 rather than to QNH.
If you round up or down too much, ATC will (should) query the discrepancy.
