The problem with the score is not that 77 or 78 is bad in a 0 to 100 scale, but the meaning of a 78 score to the game when Metacritic became such serious business that companies pay bonus if certain scores are achieved.
Reviews are always opinions, but they are also affected by external factors (publishers sending "gifts", or publishing their own "trusted review sources" to praise their own games). Besides, games from publishers and series with larger fandoms usually get better scores than the rest. For example, the chance of having a Mario, GTA or Final Fantasy fan reviewing and being more forgiving to an average game from his favorite series is much higher than other less celebrated series.
So, in a fair reviewing system, 77 would be okay and deserved, but for the average consumer, oblivious to those facts and hardly an enthusiast that hardly seek other information besides reading a magazine or visiting a gaming website will take it as bad when compared to the lots of 85, 90 and even 100 scores featured in each magazine edition or month in a review site because of the factors aforementioned. The impression a casual reader will get is that the game is average to bad with the comparatively low score and the insistence that the game is no Call of Duty Blops II or Halo 4.