Its just because you redeemed a Achievement while your Inventory was full and won a Part.
The Part wont get sold automatically in Achievements, so it will blow up your Inventory Limits
How to get 305 Aftermarket Parts or more:
1. make sure your Inventory is full (297/300) or (300/300)
2. levelup your Driver until 60, you will get "5x Maxxed License Place"
3. redeem this Award via the Pending-Rewards list
4. you are done, you will have (302/300) or (305/300) in your Inventory