Lots of factors that could be at play here, in my most humble opinion, and it appears several others.
1- The thickness of the flat usually has more influence on the cooking time as opposed to the weight. Imagine a 4 lb flat that is 1/2 inch thick but 2 feet long. I know, crazy example, but you get the point, pardon the pun.
2- Fat content. Stated above.
2- Cooking in a pan can introduce all kinds of variations. Would need to know exactly how you did them in a pan: Was the pan large with high sides? Did you use a rack inside the pan to elevate it from the bottom of the pan, etc. It makes a difference
3- Your oven temp is not accurate. I do not know what oven you have, but I would bet my left kidney and spleen that it is not accurate. Especially at low temps like 225*. My oven is not, but I know it's tendencies.
As stated above, forgo the temp as the end all for doneness. Go by tenderness and the poke or toothpick test.
Personally, for BBQ or BBQ meats cooked in something other than a BBQ/Smoker pit, I simply grab the meat, or using utensils, act as if I am going to eat the meat or product and see how it reacts. I use thick glove liners, with latex stretched over them so I can pick up a rack of ribs, a brisket, butt, etc., and see how the meat reacts when I pretend to rip off a rib and eat it, or try to pinch off a piece of brisket with my fingers like I am about to taste it. If the meat don't behave like I am about to eat it, back on the heat it goes.
**Prolly your oven aint nowhere near bout accurate. I got a pretty descent one and it is off by as much as 50 degrees at low temps, i.e. anything below 300.**