Synfig arithmatics in render / dimension fields

I’ve tried something unconventional out recently, and thought it strange for Synfig to be inconsistent on one thing.

I had a storyboard animatic in Blender, where two “drawings” equalled a Synfig scene. First drawing had 148 frames, and second drawing had 157. So…

Go to Edit > Properties. In the “Duration” bar, type in 148. It quickly takes the number as “number of frames”, like 148 f, which equals to 6s 4f. So naturally, 148 + 157 should have an equal effect, where both numbers can be taken as “number of frames”, totalling to 305 f, and hence 12s 17f.

But that’s not the case: typing 148+157 instead gives me 2m 34s 13f.

So… what’s the logic here? Why is it inconsistent?

Why don just type 305 in the duration field? I don’t believe that any of the fields are ready to make arithmetic operations, more even without units on the digits.

Well, I can type 305 – or any total – and just go with it. Here, I’m just wondering what causes it to give me a different number.