>>The other possibility of having @tax float. Since float is not a precise number, that may explain the problem.
>>If you exclude @tax condition will you always get the same rows?
>>
>>BTW, is Tax column float in the database? If yes, it's a bad decision.
>
>This select was rename at various level to hide private information. Tax is just a placeholder and represents a boolean field in the real table.
>
>This is related to the date precision that differs from the application and SSMS.

That could be true as well for the datetime column. I was struggling myself recently in our application (on the client side) with that problem.