Hey A319...
Your question would be better placed on the ATC forum -- and would get a lot of answers quickly.
Though it depends somewhat on where in the world you are talking about, the answer is generally this: If you are "positively identified", and being provided radar service in controlled airspace, the controller can issue you atlitudes lower than those you have displayed as safe altitudes on your own charts and/or displays. The reason (primarily) is that minimum radar vectoring altitudes, for example, are flight checked to a more precise standard (meaning no obstacles within a smaller horizontal area).
Hope that helps a bit...