I obviously don't know this for a fact, but it's pretty certain.... For the -1 channels, they simply DUPLICATE the guide data from the satellite-carried channel. Reason I say this is during a contract dispute earlier this year, when Dish changed the guide data for the satellite channel to their propaganda line, it was also shown on the -1 channel's guide data.
The problem is they don't in a few cases. Neutron's example (19-1) is a good one where Dish carries the local (CBS19) but the OTA has no guide
Two of my locals the issue is Dish carries them on the locals but because the OTA isn't a -1 there is no guide info. We have a 2nd PBS (think PBS Create but local programs mixed in). Its carried on Dish on 17 (which is their old analog number). Digital OTA they are on 2-3 (2-1 being the main PBS, 2-2 being the Minnesota Channel where they show programs from different PBS stations in MN, WI, ND & SD) and 2-1 & 2-2 they have guide info (2-2 isnt carried on Dish)
2nd one is KSTC 45. They are on 45 on Dish. Recently (2 months ago) the ABC (5) that owns both decided to consolidate them. So 5-1 is ABC (guide info), 5-3 is MeTV, 5-4 is This (no guide for either) and KSTC 45 is now 5-45. No guide info and I've submitted it to Zack and Mary to forward on. Also e-mailed Dish Quality to no avail