I have had similar issues with my latest Motability order.
In December I rejoined the Motability Scheme and ordered a VW Polo 110ps DSG.
Assumming no problems with regards to placing my wife and 21 year old daughter on the insurance policy I placed an order.
Whilst seeking details of insurance elegibility I discovered the following statement – “To make sure the Motability Scheme is affordable, drivers under 25 can only drive cars that meet these criteria: Petrol, diesel or hybrid cars with an ABI Insurance Group of 16 or lower. A power output of 120 brake horsepower (BHP) or less. Electric cars with an ABI Insurance Group of 21 or lower.”
During the waiting period my VW Polo’s engine capacity increased to 115ps which was the same engine used in most of the VW group small cars on the scheme and which were strangely classed as NOT accessible for under 25’s alyhough the engine was less than 120BHP. This also applied to Ford Fiesta’s and other small car brands.
With my new car now only two weeks away from being delivered the VW dealer has confirmed that my daughter can be registerd on the insurance as MOtability have recently updated their policy.
With regards to the OP I would take the matter up with Motability as the difference in thier chosen car is 4 HP which is neither here or there. It is mererly due to the manufacturer making improvements and updating/making improvements which Matability should be following suit.