Strange query results with dates

Hi!

Help me please to resolve my problem.
I cann't understand why my query returns very strange results.

select datop from mytable where date_trunc('month',datop)
=date_trunc('month',abstime '22.05.00')

it returns a strings with operations which were made in May,

but similar query

select datop from mytable where date_trunc('month',datop)
=date_trunc('month',abstime '02.05.00')

it returns operations which were made in february !

I'm running PGSQL 7.0 ,
SET DATESTYLE ='GERMAN'
SET TIME ZONE 'EUROPE/MOSCOW'

Tell me please , what I made wrong?

Thanks for any suggestions.

Igor.