accessing primary key slower than accessing non primary key???

hi,
I am using postgresql 7.1.3 under RH linux 7.2
For sake of comparison of speeds i am doing a small experiment.
I am having a table with 20 columns (1 col with bigint as primary key)
the size of a record is 300 bytes.
On retrieving records with select query with where clause accessing a 
primary key and a non primary key the times are as follows


              time in ms per query retrieval
                py key   non py key
                ------   -----------
2000            15.8     15.7
10000           71.2     70.7
25000           174      173.5 
100000          4319     3417

As u see the non primary key retrieval is faster. AFAIK accessing primary 
key should be faster. Can anybody throw light on why is this anamoly.

I use libpq with blocking execution of queries.
I ran this test with there being no other load in the system.

TIA.

--------
regards,
hari
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