I believe this was already answered in another topic, but this was an intentional change in S3D V5. We had customers who frequently told us they wanted to be able to enter the relative size increase for the part regardless of any prior scaling. For example, they might use the gizmo to resize the model a few times (perhaps ending up with some arbitrary 132.6% scale), then they would use the model transforms panel and they wanted to make it another 10% bigger. They didn't want to have to calculate 10% of 132.6, they just wanted to enter 110% and hit enter. We had enough people request this, that the behavior was changed in V5.
We will certainly keep tracking this to see if there's interest in bringing back the old behavior. Perhaps there's a way to support both modes in the software in the future.