It somewhat common in Boxster to get the gauge stuck. Some people have a gauge that reads 1/2 full, anywhere in between a full tank and 1/2 tank. Then below the 1/2 tank mark, the gauge begins to work and read to true amount. In your case the gauge is not moving past 3/4.
You can probably get it fixed, but I would just do the OBC hack then you will always have a very accurate reading, and it only costs about $50 to do.
