My bet is on this being a measurement error, or an extra force they didn't think about.
I e-mailed the main guy behind the paper and pointed out that they too easily disregard the possibility of this being an electrostatic effect because they say that can't be the case because the setup is "grounded". The air quotes are because I'm working on an electrostatic instrument, and our experience is that it is very hard to remove all surface charges. At the kind of forces this paper talks about it doesn't take much charge buildup for some nearby electric field to contribute force via Coulomb attraction. Any kind of grease stain or oxide patch can do this