I'd wager that the load T/C assumes you will use for the .45-70 is the 300gr HP factory ammo at about 1800-fps. My Encore Katahdin .45-70 will NOT zero with factory 405s, but no problem with 300s at 1800-2000 fps.
What you are experiencing is indeed bullet impact change due to recoil. I know, I know, 50% of people believe that the recoil doesn't affect bullet flight. Well, it does, especially at the low speeds of .45-70 loads. Your 300gr loads will zip out much quicker, while the barrel is still lower in its recoil track. Thusly, they will go to zero. Now, toss a bigger, slower chunk of lead, and your muzzle is further elevated at exit (more barrel dwell time). Presto! 18" high. My .45-70 barrel will throw 405s about 18-24" high at 50 yards when zeroed with 300s.
That, and I think T/C has a hard time figuring out how tall to make their front sights. My .50-caliber muzzleloader barrel shot about 6-10" high at 50 yards with a 100gr load and a 240/250 sabot. Of course, it went to zero with the faster 150gr load, but I don't want to put up with that kind of kick.