It is, of course, very difficult to raid with more than one group. Heck, sometimes raiding at all can be a very difficult proposition, requiring you to set aside hours of time. So it becomes difficult to even see other groups in action. You can watch videos, yes, but in many cases the videos themselves are recorded by groups that are themselves above the average, with players who are skilled and knowledgeable about their classes and roles and about raiding in general. When one raids with a group, it becomes easy to assume that said group is how the game is played and even easier to forget that many players can't raid at all - the raiding game is itself a fraction of the player base, and the 'average' player may not even see the inside of a raid above the LFR difficulty. Indeed, the introduction of LFR and Flex has, to some extent, gotten more players into raiding than ever. And this to a degree inflates what we're discussing when we use the word 'average', because if the average player is running LFR and you're clearing normal mode, that puts your group in the above-average category, even if you keep wiping on Thok over and over again.
It's this inability or unwillingness to imagine outside of the personal experience, to recognize that each group is comprised of individual players with different experience and each group itself has different experiences that leads to this difficulty. Seeing what the average even is requires one to look at multiple groups - how many are clearing the content? How many are half-way through it? How many never even see it? If more than 50% never get past X boss, and you have gotten past said boss, you are not average.
A lot of players seem to believe that, because they're not in a world-first heroic guild, they're somehow terrible players. This is an understandable conceit. After all, those players are obviously some of the best players. But using them as a measuring stick is an awful idea. First off, they are the definition of 'above average' taken to absurdity. They play not only on a different level, they approach how they play differently. They are willing to spend much, much more time up front to clear content fast. They will min-max their raids, switching in the absolutely optimal class/spec combinations. Many of them have multiple alts ready to raid. They often work tirelessly on strategies to compensate for the fact that they're doing the content in gear that is significantly behind where other players will be when they do the content. Not being those players doesn't make you a bad player - it doesn't even make you a below average player. You can be an excellent player and not be one of them, for a variety of reasons - schedule, commitment, even a lack of interest in investing the time and focus necessary to raid at that level.
Just as using those players as a baseline to establish what the average is comes up short due to how far above average they are, using your own group to establish what it is can be a flawed proposition because, while you may or may not be at that level of play (I have no way of knowing) you certainly aren't representative of the game as a whole. Your raid group is simply an insufficient sample. You are not the game. You are not all players, or even close to it. If you are successfully raiding at all, you're probably at least slightly above the skill and dedication of the average player. It's worth keeping in mind - if you use tools like Wowhead, read sites like this one, check on developer's twitter feeds, you're already more invested than the majority of players.
The average is amorphous in this context. It's not the mean or the median, not something you can just tally up and say 'this is what it is' because you're simply not working from the right pool. You don't experience the average - you are most likely already above it.