Basically, there is a difference in build-time calculation for chroots and builds. For chroot, it is end_time - start_time and for build it is chroot_max_end_time - chroot_min_end_time (in other words, for a build, all its chroots are considered in time calculation).
The thing is that your x86_64 build got stuck and was restarted after 48 hours at which point chroot's start_time got reset. You can find out where it got stuck from the logs prev_build_backup subdirectory in the build's results directory.
It's quite common that kernel (or long-running) builds get restarted. I am not sure why exactly it happens and if we should take a look at it. Let us know if the problems are reoccurring.