It is a good point, and what is no doubt frustrating to the timetable planners is that the plan is constrained by a 1-2 minute issue on paper, when in practice many services suffer 10-20 minute delays or more regardless of the traction type. Unfortunately the rather more 'tolerant' days of 'don't worry it'll work on the day' are no longer with us, and the plan must be 100% compliant otherwise NR will not accept it. Sadly the industry sees a 3-car 90mph unit which can keep time but suffer overcrowding as entirely preferable to a 5-car 75mph unit which can't quite keep time but provides seats for all. Such are the times we live in, and I'm sure there are many ex-railwaymen in the sky looking down at us all and laughing their backsides off.
It's all about liability you see.