Most likely, it's as you described, router N forwards packets much faster than it generates icmp ttl exceeded, and router N+1 is nearby and generates icmp faste

Most likely, it's as you described, router N forwards packets much faster than it generates icmp ttl exceeded, and router N+1 is nearby and generates icmp faster. However, it could also be the case that the routing back to you is significantly different, so you can have a much longer path to you from router N than router N+1. This is more likely to happen on routes that cross oceans. Say you're tracing from the US to Brazil. If router N and N+1 are both in Brazil, but N sends return packets through Europe and N+1 sends through Florida, N+1 returns will arrive significantly sooner.