Why does service differ on the day you travel?

As a regular traveller from the Carte Blanche gang on Eurostar I'm wondering why the service on board differs and depends on the day of the week you travel or the mood of the crew on board. Last week my outbound journey on a Thursday afternoon was a joy and everything that made traveling itself a more than pleasant experience, rather than a necessity in this day and age of cattle carriage class on 'not-so-cheap-anymore' flights. The team serving us was courteous, friendly and well trained in the art of customer service, making sure that each individual was greeted with a personal touch to lift the spirits and with that rare quality of putting the comfort and well being of the customer first above the usual don't ask me I only work here mentality. While on Sunday on my return journey, and you know where this story is going, the crew was clearly peeved off having to work on Sunday. Now I do understand that working on a Sunday evening is not everyone's idea of a good time, but its part of the job and came with the uniform. So instead of getting great service it was ticking the boxes and hoping that no-one on board was too difficult or needed more service than absolutely necessary to fulfill  the minimum effort. While on Thursday the team couldn't do more to please their onboard customers, the Sunday crew almost disappeared when the obligatory one cup of coffee or tea was served. Just to illustrate the ignorance of the crew was the main course served, which, apparently only in my case maybe because I made a remark, wasn't what the menu stated. How the server didn't notice that the salmon he served me wasn't with ratatouille and wild rice, but with endives, potatoes, green beans and cream sauce is beyond me. On its own this is not a major drama as the salmon tasted good, but shows that either the crew couldn't give a ... or there is a clear need of staff training in the art of serving.  So clearly service differs on what day you travel and depends on the mood of the crew. My question is however why am I paying the same price for both journey's as it doesn't reflect the difference in level of service?  Not that I'm pleading for cheaper travel for poor service, but exactly the opposite: deliver the same great service regardless of the ticket, day of travel and above all the on-board crew. You know you can deliver, but not on every day apparently ...