Bug in masterworking?

Hello dear support! I have a problem with masterworking. Im trying upgrade my fractured winterglass and even if 2x 25% bonus goes to conjuration mastery its still +3. How is this possible? Its a bug and it will be fixed or its normal? If i compare to other similar thing for example hellbent on amulet: start is same +2 without greater affix. And hellbent is +4 at the same masterworking point.

Thank you for answer.
Thank you in advance
Teranti