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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