On several occasions when I have corresponded with FGW , I have been given the following response :

"That was the specification that we bid against."

The link below contains the specification on which tenders were invited for the Greater Western Rail Franchise.
http://www.dft.gov.uk/foi/responses/2007/may2007/greaterwesternfranchise/greaterwesternfranchise

I have noticed something very interesting (bear in mind that the number of calls mentioned are required to be made in BOTH directions) :

"Dilton Marsh

(a) On Mondays to Fridays between 0600 and 2359, 10 services shall call at a maximum interval of three hours.

(b) On Saturdays between 0600 and 2359, 10 services shall call at a maximum interval of three hours.

(c) On Sundays, four services shall call."

Quote from Peter West (DfT) :

"The minimum contractual level of train service is set out in the FGW Service Level Commitment 2 ('SLC2'), which forms part of the FGW franchise agreement.  Some changes will be made with effect from December 2007 to the route within the SLC2 of which Dilton Marsh forms part when the SLC2 reverts to its ORIGINAL form following the transfer of local train services south of Salisbury to South West Trains.   

For services on Mondays to Fridays and on Sundays, the Department does not plan to reduce the contractual minimum level of service require for Dilton Marsh post-December 2007 compared to the contractual minimum level of service provided for now (5 train services each way - Lee.) On Saturdays, the contractual minimum level of service will reduce from eight calls at Dilton Marsh in each direction to five in each direction.  The SLC2 has not yet been rewritten and the above is subject to final confirmation.  The numbers of services shown are minima and do not contractually prevent FGW from offering more stops if this is possible operationally."

In effect , the DfT have only specified half the Monday - Saturday calls at Dilton Marsh for December 2007 , compared to the original SLC2.

A view from Alison Forster :

Thank you for your e-mail and your patience while waiting for my response. The specifications posted on the DfT website were the minimum service specifications for the Greater Western Franchise, showing the services that we had to run. We made over 200 changes to this specification to run further services but there were more changes that we were not able to make for both financial and practical reasons.

We recognise that there are more changes that customers would like to see and we will continue to lobby for those changes where we feel that they are appropriate. I appreciate you taking the time to contact me and I hope that this helps to clarify matters.