mk-sbuild by default for ubuntu and debian schroots runs debootstrap with the --no-install-recommends to keep the generated chroots as minimally installed as possible. However, for ubuntu schroots, the resulting chroot is still configured with install-recommends enabled in its apt configuration:
$ schroot -c trusty-amd64 apt-config dump APT::Install-Recommends
APT::Install-Recommends "1";
This results in a schroot that differs in build dependency resolution behavior from the buildds, which has install-recommends disabled by default. I noticed this difference when a local sbuild succeeded because a (recently split out) recommended package got pulled into the chroot, but failed when built in a buildd.
As long as the buildds don't use sbuild from the archive, there's always going to be (the possibility of) some differences in dependency resolution; however, it seems wise to try to minimize those differences so as to give developers an environment that matches the buildds as close as possible.
Here's a patch that addresses the issue for me:
$ schroot -c test-recommends
APT::
I didn't expose it as a command line argument, though I could add it if it's desired; mk-sbuild already has a non-trivial amount of possible arguments, so my initial instinct was to resist the urge to add another to the pile.