My battery capacity just decreased when i installed manjaro xfce before i was using kde neon

$ upower -i /org/freedesktop/UPower/devices/battery_BAT0 | grep capacity
capacity: 73.7514%

my kde neon was having 80 capacity why does it goes down

Generally speaking you can expect those capacity percentages to be innaccurate until you do a full power cycle or two. In the vast majority of cases this will result in a lower percentage than previously reported.
I dont know if this is the case here … but it seems quite possible.

The percentage is calculated by dividing energy-full and energy-full-design then multiplied by 100, and the dynamic one is energy-full. This may vary as you may not start from actual 0Wh capacity to boot so a several power cycle is required to have a somewhat accurate representation. Really to make it extremely accurate is hard, as degradation still happens during the phase of stabilizing the result.

I’d say coincidence and as already mentioned it is not possible to calculate accurately - wear and tear happens to the battery all the time - upower is the same everywhere - so I understand your question.

I would likely think the same - expecting the battery condition to be the same - but upower cannot report anything other than what the system battery reports when queried so - pardon the phrase - bad luck?