The difference here is that Bayer Leverkusen are capable of handling most of the big teams in the bundesliga whereas the same cannot be said about Frankfurt and at such it would be very easy for the Leverkusen side to suddenly overtake Frankfurt soon enough as the season progresses. Frankfurt's inconsistency will be a determining factor and an advantage for Leverkusen who might take advantage of Frankfurt's mistakes this season to go above them because as it is right now in the bundesliga, the only team that can compete favourites against Bayern Munich is the Leverkusen side.
I agree with you, we cannot be sure that Frankfurt can compete with Bayern Munich this season based on their current performance, in my opinion. Frankfurt has recently lost their form and isn't giving their opponents as much trouble as they did at the beginning of the season, in the previous match against Augsburg, they struggled to get a draw, despite the opponent's 13th place in the league. However, after their achievements from the previous season, Bayer Leverkusen are performing well, but they are still seven points behind Bayern Munich, which will be very difficult for Xabi Alonso's team to reduce before the season ends.
Furthermore, Frankfurt's next opponent is Leipzig, and I am confident that it will be a difficult game for Frankfurt to win. Despite Leipzig's inconsistent performance this season, I doubt they will lose at home against Frankfurt, whereas Leverkusen will play Augsburg, and if Leverkusen wins, it will have an opportunity to overtake Frankfurt for second place. But has anyone observed Wolfsburg's incredible performance this season? They are presently sitting 5th in the Bundesliga table.