United is obligated by its own denied boarding policy and by US Dept of Trans. regs. to first seek volunteers before involuntarily denying boarding. UA's invol. denial rate is small because it does strongly prefer VDBs and seeks to get them, witness your own experience. ORD has so many connecting pax that a flight that soemtimesappears that will undersell due to the likelihood of misconnects may oversell at the last minute when the pax actually show up running from C21 to B3. Federal rules do not require IVDB compensation if UA can get the pax to her destination within an hour of sked. time and there are some large cities out of ORD for which that would be possible. Also, no IVDB comp is due if the pax holding a confirmed res. checks in after the cutoff time and that is what you may have seen. From that selfish persepective, it's cheaper for UA to IVDB in those cases or in cases of flights with many cheap tix, (ORD->MCI, say) as IVDB comp is based on ticket price. But UA must at least ask for volunteers before IVDB'ing pax and there is the issue of goodwill. The new video gate displays at ORD have replaced the older VDB signs that used to sit on top of gate c/i podiums when agents were concerned about an oversale.