Weird query speed

Almoust identical querys are having very different exec speed (Postgresql
7.2.4).

query: select "NP_ID" from a WHERE "NP_ID" > '0'
Index Scan using NP_ID_a on a (cost=0.00..13.01 rows=112 width=4) (actual
time=16.89..18.11 rows=93 loops=1)
Total runtime: 18.32 msec
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query: select "NP_ID" from a WHERE "NP_ID" > '1'
Index Scan using NP_ID_a on a (cost=0.00..13.01 rows=112 width=4) (actual
time=0.08..1.36 rows=93 loops=1)
Total runtime: 1.56 msec

From where such difference comes?

There are about 37K rows and only about 100 of then are not "NP_ID" = 0

For a workaround i use WHERE "NP_ID" >= '1' and if works as speedy as '> 1'

Rigmor Ukuhe
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