Interesting. I'll definitely give this a follow.
Best way I could explain it is that it is always comparing calendar months.
This is the Switch profile for November, Switch has been out for 3 years & 9 months. For LTD of 70.91m
For Switch that 3 years & 9 months comprises the following:
4 x March, April, May, June, July, Aug, Sep, Oct, Nov
3 x Jan, Feb, Dec
Then the gaps table compares that LTD to the other consoles using the same 3 years & 9 months, but it's based on those calendar months, so for DS the LTD is the first 4 March months + first 4 April months etc..... but only the first 3 for Jan, Feb, Dec.
It's not strictly linear time as it's not including DS's 4th Jan/Feb/Dec even though they happened before DS's 4th March.
After the initial launch period sales follow a predictable pattern over the calendar year so this is a different way of showing how consoles are doing. It's one way of showing the better momentum the Switch has at the moment over the Wii for example, in the normal launch aligned chart as in this article the Switch is still behind the Wii. A little analysis and you can see it is clearly going to pass it in December but this gap table shows it as ahead by 7 million already, in other words if it were to drop to Wii sales levels for the rest of it's life it would still end up ~7 million ahead.