No, all the time periods go backwards from the same starting point - i.e. today. And each time period is compared to same preceding time period for each of the clubs concerned - e.g. the last 6 years for Spurs compared the preceding 6 years for Spurs, or the last 5 seasons for Chelski compared to the preceding 5 years for Chelski.
The trends are clear, however much you try to quibble about it.
Yeah I know they're starting from the same point but they're not the same length. Like if you start today from London you can go to Beijing or you can go to Dover and it won't be the same distance.
Why is it 6 seasons for Spurs but only 3 for United? Because it's the only period of time you can possibly come up with that makes United look poor and makes Spurs look solid? If you take 4 years for United then you're having title winning season so you cannot do that. If you have anything below 6 for Spurs then this year is the only time you make CL. That's the reason, right?