[Elecraft] K3: DSP filter steps; am I wrong?

[DOUGLAS ZWIEBEL]

HI all...

I really wonder why there is so much discussion about the filter step
size, which is currently 50hz.  Maybe I am just nuts, but doesn't that
50 hz "come off" of each side of the BW?

Here is what I imagine...

I am on frequency A with a DSP filter BW of (say) 500 (lets ignore the
roofing filter for now) and I hear a little QRM a bit above me.  When
I crank the DSP down to 450, isn't that moving the "leading edge" only
25 hz (and also the trailing edge).  So in reality, am I not
"changing" things in 25hz steps in terms of the offending QRM?   Is
there really a situation where you would need to narrow it 12hz
instead of 25hz (and appreciate the difference)?

Maybe those requesting a smaller step need it for PSK?  But even then,
it doesn't make a whole lot of sense to me.  How often would adjusting
the DSP bandwidth something less than 25hz (since that is what we have
now), be better than just going to "full" 25hz?  Or viewed from the
other direction, if you want to narrow it even more, narrow it "one
more step" and get really narrow.

If I understand this correctly, great.  But if I don't, please fill me
in because I would genuinely like to find out more.

Thanks!!
de Doug KR2Q