SQL Server RandomNumbers Stored Procedure Error
Because you evaluate @X and @y once before the update. 

You need to do in the UPDATE itself

Update driver_tbl 
set 
    pos_x = ROUND(RAND(CHECKSUM(NEWID())) * (@maxvalueX - @minvalueX),0) + @minvalueX,
    pos_y = ROUND(RAND(CHECKSUM(NEWID())) * (@maxvalueY - @minvalueY),0) + @minvalueY


The answer I gave on your SO question shows this: https://stackoverflow.com/q/8546985/27535