Hodgdon does not manufacture any powders. They are a distributor.
Some "H" powders are made in Australia by ADI (Australian Defense Industries), especially marketed as "Extreme" and are "temperature insensitive" to a certain point, anyways, less sensitive than most other powders.
IMR powders are made in Canada (General Dynamics in Valleyfield, Qc) and are about the same as when they were made by DuPont.
Now, as history goes; Hodgdon bought large supplies of ex-military gun powders and these were sold by their original names; i.e. say 4831. These powders were not that young and have slightly changed burning rate from aging. When his stocks dried out, Hodgdon had it "copied" by another manufacturer (not DuPont), then, they copied the actual powders, not the original recipes / propoerties of the old powders. He then kept the H-4831 name.
When DuPont saw the advantage of selling powders to the general public, they re-introduced the "original" (well, at least as close as they could) powders, they used the original name, hence the IMR 4831 powder.
As a conclusion; IMR and H powders, while close, have different burning rates and can't be considered the same. That's why Hodgdon kept both of them. Both brand of powders are excellent products, as long as you can live with their own properties.
Personally, in some cases I use "H" powders (my varmint guns, as an example) for use in cold weather or very hot temps, but usually I don't care for my big game rifles.
Just remember; never develop a hot load under cold weather, with whatever powders. Even with "Extreme" powders, you may have a surprise if you shoot these loads in a torrid summer day.
