Why of 1990 Levels?

It is reported that the EU have finally resolved to cut their 'greenhouse gas' emissions by 55% before the end of 2029 compared to 1990 levels (https://www.theguardian.com/world/2020/dec/11/eu-leaders-reach-deal-to-cut-emissions-by-at-least-55-by-end-of-decade). It has, apparently been difficult to get agreement (although the EU is said to want to be a leader in the fight against climate change), because of resistance from their Eastern nations. These countries (e.g. Poland) are currently very dependent on coal for power and would find the green transition more costly (I expect they have been arging for more funds to do this?). One might ask why the reduction in emissions is linked to 1990 values? 1990 was 10 years ago and levels of emissions have increased since that time. Was the choice simply to make the 55% look as big as possible? Of course, after 1st January 2021, the UK could choose to make much more drastic cuts. I think this is unlikely (and, anyway, I currently don't trust any government's figures on emissions reductions).