locking a single row with pointbase???

Hi!
I am programming transactions in TRANSACION_SERIALIZABLE mode. At the
beginning of a transaction i want to set a write lock on a single
table row with this UPDATE command:
this.stmt.executeUpdate( "UPDATE TABLE SET(COLNAME) = VALUES(anything)
WHERE COLNAME = anything);

In TRANSACION_SERIALIZABLE mode the pointbase DBMS locks this row and
the following row in the table also, that means 2 rows??? i dont
understand that.
In TRANSACTION_READ_COMMITTED pointbase only locks the row which the
UPDATE statemant applies to.

Has anybody an idea why pointbase does this???
thanks
Holger
.