In this instance, MSO stands for Microsoft Office. My version of Excel, Work and Project all have the same version after MSO (14.0.6123.5001) – which is different than yours, but then all my Office 2010 is at SP1 – maybe that is the difference here. Office applications share common features, and maybe this MSO level identifies the level of common features?
SP1 (Service Pack 1) is installed on some of your PCs, easily identified by the SP1. SP1 may have been installed as part of Microsoft Update. Ideally, all your users using Project (and the rest of Office indeed) should be at the same Service Pack Level. There are IT tools to automate this.