I will never understand why some people say this and that top group flop

Raw numbers mean nothing to success without context of cost and budget. You can sell 2 million albums and still it would be a failure if it cost way more than what that 2 million brought in to make.

Because the maybe different people have different definitions of the word "flop"

I'll say what I said about "flop" previously

also there are many definitions of the word "flop" - for example SK charting wise

1. to not chart at all - fairly obvious that if one fails to enter the charts at all then it's a flop

2. to not chart as well as previous songs - this is fairly understandable that if a group had a PAK for one song but the next song only made it (let's say to the top 20) then one might argue that since the song didn't do as well as previously then it's a flop

3. to not chart as well compared to their peers - the question who one's peers are is important since charting is by it's very definition about who one is higher or lower than...some people will say if x charted worse than y then x is a flop

4. to not chart as well as expectations (this is the most subjective) - here a group flops because there is an expectation that certain groups will perform well whether based on prior success or company reputation...this generally applies to the larger or more popular groups - ie. if IU failed to chart (maybe) in the top 5 then one might argue that because of IU's prior successes then her level of expectations is much higher than a lesser known artist or group...

Some people just say it to be hateful. These should be ignored.

It matters in comparison, usually, like some people already noted.

A big 3 group that never makes it higher than 80 on any relevant charts can be seen as a flop.

A very nugu group with almost no budget making a song that charts at 80 will be over the moon with that result.