Keep in mind when using that function it is not equivalent to group levels but rather times drilled down into. You may have 5 levels of grouping. If you don't have the data suppressed you can start drilling into the report at group level 5. If that is the first time you drill down then your drilldowngrouplevel = 1.
On initial opening the report the drilldown is at 0.
Sometimes you need to alter the setting for "show all headers on drill down" to get the results you want.