In addition to @kevinushey’s good suggestions, maybe verify that you’re running in a new session and that none of these machines are loading anything from an .Rdata file on startup? In RStudio, make sure “Restore .RData into workspace at startup” is turned off in Global Options and in Project Options, if you’re using a Project.
Maybe also check that there aren’t any commands hiding in a startup file that could be altering things?
Finally (or maybe initially!), if you examine ab$Month_considered directly, does it look right on both machines? What code is creating it, and can you verify that that code is running successfully?