Different query plans for the same stored procedure against different logins

Hi,

It has now been few occurences where I see different query plans for the same stored procedure against different users. The case is somewhat like this:

A stored procedure is called by two database users - one regular user with 'exec' permission and another sysadmin. Both execute the same command like exec SP1 Parm1, Parm2 with same parameter values. However the instance executing under regular user has a different query plan than the other and takes longer.

I was wondering what could be the reason behind their having different query plans.

Thanks in advance.

Nirupam