Hah. There are always a lot of moving parts in espresso brewing.
Hard to say for sure, but I think your theory that the beans hit the right age is probably correct. I say that because with the 20:40 dose grinding coarser would normally extract less, and hence taste more sour.
One thing that happens with aging is CO2 outgassing. CO2 can block hot water from contacting the grounds, interfering with full extraction. So when the CO2 level drops you can extract more.
Another thing that happens with aging is that the beans get more brittle, which tends to create finer particles at the same grind setting. So more gets extracted.
With lighter roasts, both these factors add up to better extraction if you let the coffee rest longer after roast. With darker roasts, you typically have to grind coarser as the coffee ages.
Not to get too picky about language, but I prefer to use the term Updose to refer to the technique I described above, which is a method for minimizing bitterness in dark roasts, and often requires increasing the brew ratio and brew time. Back in the day, updose amounts could get extreme -- 22g-25g, with brew ratios of 2:1 or higher.
That said, some people may use the term to describe anything larger than a 14g dose. In that case, you were already updosing at 18g!