The difference in BTU ratings you note is a result of different test protocols. The EPA test laboratories are concerned only with particulate emissions, not maximum heat output. In fact, the EPA label that comes inside every woodstove has a disclaimer on it, stating the EPA lab didn’t test the stove for heating efficiency. Here’s why:
The only way the EPA can test emissions fairly is to burn the exact same load of fuel in all tests. Same shape, same size, same weight, same moisture content. To accomplish this, they use nailed-together “charges” of kiln-dried pine in a size and shape that will fit any woodstove.
These “charges” are fairly small, and contain far less wood fiber than a full load of conventional cordwood. Thus, when the EPA lab tests a woodstove for emissions, their charge produces much lower BTU output numbers than if they’d filled the firebox with cord wood (hence the disclaimer). This is true of all woodstoves, but especially evident in woodstoves with large fireboxes.
In addition to the EPA testing, a woodstove manufacturer may elect to take the stove to an independent test laboratory for heating efficiency testing. These tests are performed with full loads of wood, and produce a real-world “Cord Wood” BTU output that makes them look much more powerful than the stoves whose brochures only give the output produced by the EPA charge.
We had a look at the EPA label on the woodstove whose brochure stated a Cord Wood rating of 79,000 BTU/hr. The EPA output rating for that model was only 40,000 BTU/hr., about the same as the 42,000 BTU/hr EPA output rating on the other stove you’re comparing it to.