Share locks in uncommitted isolation level

[htanguy]

Hi,

I have the follow strange situation :

isolation level : uncommitted
autocommit off

Transaction 1 modifies a table row R1:
Executes an "Update" request on R1 => produces an exclusive lock on R1.

Transaction 1 not ended.

Transaction 2 begins and reads the same table :
Executes a "Select" request on this table => sometimes this causes shared lock on R1.

Why a shared lock is put on the read row in uncomitted transaction level? Why is it not always repeatable ?

Thank you