Well, the +10 window is for variation in 'real-life' taxi time, other causes of operational friction, and finally ATC sequencing at the holding point. Operators should conduct all planning on the CTOT, not CTOT +10. That's what leads to poor slot adherence and ATC capacity issues.
In the Spanish example, who is to say that nearly every outbound didn't have their own CTOTs? Maybe they had additional restrictions in place too? Perhaps some taxiway closures that made the 7 minutes until the CTOT unrealistic? Perhaps there were aircraft pushing back/about to push back nearby that would delay this aircraft's own push back? You really don't want to start an aircraft only for it to not make the CTOT, because then you have an aircraft out near the ruwnay with what might turn out to be an hour's, or more, delay with all the increased workload that entails.
Could that aircraft physically have made it to the runway in 7 minutes? Possibly. Perhaps only if every other aircraft on the airfield was stopped to give way to it. I've denied start to many flights before in this situation, as an ATCO you have to balance the need of the few against the need of the many. Sometimes your judgement is that it isn't worth it. You could stop everything for 5 mins to get this one flight out on it's CTOT, but nobody would get anywhere if we treated all flights like this.
Where I work, I'd be very unlikely to give start to an aircraft 7 minutes away from the CTOT unless it was dead, i.e. no more than one at the holding point.
Each airfield will have it's own taxi time, and if the aircraft does not call ready CTOT -taxi time then technically, yes, it is the operator's responsiblity to send a DLA message and get a new CTOT, not ATC's. Granted, ATC usually do it in my experience, but that's what the rules say.
If you call early, yes, we will use the -5 mins, but that is sensible, because there's the window after that CTOT -5.
At the end of the day, you cannot compare two completely different airfields in what would probably be two completely different traffic/delay scenarios.
That was my point.