I missed a comma out between That, and the next word, whilst doing it on my iphone, and I had referenced your statement above. Still, if you now wish to have a more considered response...
Some bus industry professionals at Carousel, with a mass of operational data and local experience, have been very busy in developing the business over the last year. They have worked with Bucks CC (and other local authorities) to develop the local bus network and are making a decent fist of it, supported in part by s.106 and BSIP funding where appropriate allied to commercial initiatives.
They have taken a view as to the best and most appropriate use of resources. That equates to understanding the impact on overall usage by the changing of the shoulder or peak frequencies, with cost benefit analysis against placing that resource into the service pattern AND against an ongoing scarcity of drivers (though this is much improved compared to the nadir of 2022). As
@Dwarfer1979 states above, it's not about the marginal cost of a driver but the full cost of that resource (and I would suspect that Carousel's costs in that part of the world are considerably higher). As it is, Carousel are (like many operators) having to battle traffic congestion in the peak and adjusting resources/headways as it is e.g. an extra five minutes into every journey between Wycombe and Beaconsfield in the peak.