Timecode issues, sort of. "TC2000 vs TC4000"

I'm noticing that if I program my show to an audio track and then sync it to timecode if I use the TC2000 it syncs up exactly. If I use the TC4000, the midi timecode reader, the timecode drifts out of sync. This is the case for any midi timecode reader I use any other midi timecode interface the timecode drifts with those as well. It's not that big of an issue other than if I program something for someone else now I need to know if they are using a TC2000 or a midi device. Any ideas why this is the case?