Playing through all five speakers does not mean that you have a Dolby Digital 5.1 mix. Dolby Pro-Logic (which is encoded in two channels) will also play through all five speakers, even if the receiver is an analog-only unit which is totally incapable of playing a DD 5.1 track. (Three semi-independent channels for the front three speakers, a single mono "surround" channel for both rear speakers, are used in Dolby Surround and Pro Logic.) Because what amounts to four channels is matrixed into two, there is some bleed-over between the channels. DD 5.1 has six separate audio channels (the sixth is the low frequency effects channel, which handles low bass.) They are totally isolated from one another.
When I was shopping for a DD receiver the salesman demoed The Lion King for me on laserdisc. (This was before DVDs, so LD was the only format that offered Dolby Digital 5.1.) First he played the Pro Logic track, and turned off the speakers one at a time. When he stopped only the center channel was playing. On the closing theme I could hear Elton John singing along with the orchestra. Then he switched to DD 5.1, and went though the same process. When he got down to the center speaker the only thing I could hear was Elton's voice. No music. That's discrete 5.1.
TG and ItB are Dolby Pro Logic tracks, which is how they were mixed for television. (Since, at the time, there was no digital broadcasting and therefore way to transmit DD 5.1 over the air.) They are encoded on the discs as Dolby Digital 2.0, because the DVD spec requires that all DVDs have a DD track. It is the standard sound format for DVD. But Dolby Digital can be anything from pure mono (DD 1.0) to 5.1. (And the more recent DD EX version can be up to 6.1 channels, adding a "center rear" channel.)
So the discs are "stereo", being encoded as 2 channel and playable on a stereo system. But a Pro Logic or DD receiver can decode the matrixed signal and play through all five speakers. It is even possible that the first two X-Files seasons are in Dolby Pro Logic or Dolby Surround, and just not flagged properly for your system to recognize them as such. (I honestly don't remember how those seasons were mixed.) You might try manually setting your receiver to Pro Logic and see what happens.
If your player is not connected to a DD sound system via a digital connection, it will automatically down-mix a DD 5.1 track to analog Pro Logic. Some discs include a separate Pro Logic surround track in addition to the mandatory Dolby Digital track, because these are supposed to sound better than downconverted Dolby Digital. But space considerations or multiple commentary tracks often make this impossible.
Regards,
Joe