C182 pilot set destination altimeter setting which is quite different from the enroute altimeter and is challenged by an SCT Controller for being 300 FT low.

Narrative

Departed for Brackett (POC) VFR. After clearing the mountains; was descending to 6;500 FT and asked SCT Approach for a practice VOR approach into POC. Was assigned a beacon code; given the ONT altimeter setting - which I put in the altimeter -; and told to descend and maintain at or above 4;500. I read back; 'Out of 6;500 for 4;500' and descended. On the way down; SCT asked if I had the ATIS at Brackett. I told them I did not; so they told me to get it. I did; and the ATIS gave me the Brackett altimeter setting which was quite different from Ontario's; which occasionally happens. I put the new altimeter setting in the altimeter. I leveled at 4;500 FT as I had been instructed. I was surprised to get a call from SCT telling me that I had been cleared to 'at or above 4;500' and that their radar showed me at 4;200. They gave me the Ontario altimeter setting again which I put in; and climbed 300 FT since they had said they read me 300 FT low. I climbed 300 FT (which showed me at 4;600 FT on my altimeter) and continued. The rest of the flight; terminating in a VOR approach at Brackett was uneventful.This was caused by a combination of factors: First; I put in the destination altimeter setting when I should have kept the enroute one; second; it happened that the 2 altimeter settings were significantly different; and third; my encoder must read on radar a little lower than the altimeter does - apparently 100 FT - which is within acceptable tolerance. The pitot static system has been checked within the last 24 months as per FAR's. I will mention the discrepancy to my mechanic and see if that can be rectified at the next pitot static check.