The looming Clasico has inconvenient timing for Barcelona. Simply put, it's a higher priority for Real Madrid, and the schedule has been kinder to them.
First, Barcelona have a 9.5 point lead over Atletico Madrid in La Liga. Losing this match would be a setback, but Barcelona would still come out firm favorites. True, Real Madrid have little to gain, either, but they're only competing in one other competition, whereas Barcelona are competing in two others.
And thanks to a very generous draw, Real Madrid's opponent is Wolfsburg in that other competition, the Champions League. With all due respect to the German team, they were considered the weakest team left. It's no gimme, but Real Madrid has a lot more margin for error.
Meanwhile, Barcelona drew Atletico Madrid, arguably one of the top four teams in the world. Atleti are also a very physically demanding side. What this all means is Barcelona may be more motivated to conserve energy than Real Madrid.
This is then further complicated by the heavy minutes Barcelona players have logged. The Blaugrana, by virtue of playing the Spanish Super Cup, the European Super Cup, and the Club World Cup - in addition to playing all the two-legged cup ties whereas Real Madrid only played one Cup match - have played way more games.
Then there was the international break, in which the likes of Javier Mascherano, Lionel Messi and Luis Suarez traveled across the Atlantic to play two matches. Meanwhile, Gareth Bale and Karim Benzema didn't feature in the international break. Cristiano Ronaldo played one match and some of the other, all without traveling out of Europe.
Because of fixture congestion, the international break, and the sheer number of games Barcelona have played, it is the Catalans who may seem more likely to - even subconsciously - leave some gas in reserve.
Of course, Luis Enrique's men seem to be winning even when they - consciously or not - don't go 100% for 90 minutes. We saw this happen against Atletico Madrid earlier. There's no reason to think Barcelona won't be brilliant, even if they hang back a bit...