[3onthetree]

The reason is the excavation overdig.

• The shallow porch is within the basement's overdig, and you can't levitate the porch foundation pour in the air, so it will be a full basement story pour. It doesn't matter structurally, it is only about methods of construction. Waiting for backfill then pouring the porch later would just settle with the backfill and extend the foundation pour project timeline.
• The deeper porch is beyond the basement's overdig (that example drawing may not have a basement anyway, and the porch is shown as a trench, but for discussion it has a basement and porch footings). So the porch foundation can just go to frost depth, and the part of the porch foundation wall will be bridged into the basement wall (haven't seen it done, but it appears the "CMU Lead" is filling under the bridge with CMU).

Here is the excavation in RED for the full story basement, in BLUE just to the frost depth:

 

[3onthetree]

then it's good and they go 42" below after the basement is backfilled for porch footings

and as you stated there is a 42" cut after the fact so porch can be trenched out after and either poured or cmu block

No, and I think you are even misunderstanding what Matt1963 said. You do not backfill and then re-excavate later for the porch when you have frost footings. Everything is dug at once, porch and all. A "bench" cut, if used in excavation, has to do with the earthen walls caving in, not the foundation. If the upper tier (the "bench") happens to be at the depth of the porch frost line, so be it. Whether you do a bench cut, or the angle of repose has to do with the type of soil you are digging in.

 

[3onthetree]

That drawing markup is not to scale, it was just for discussion, so I don't know how far out an excavator at your site would choose to dig. I assume that is not your house anyway because both drawings of house designs are so different.

So here are a couple interweb photos that might help put this to rest (they are elusive photos googling with the word "bridge"). There is a gap in the overdig under the higher elevation footing - and this is where the "CMU Lead Wall (Typ)" comes into play, I believe they are saying to infill under the bridge with CMU after the pour has been stripped ("Typical" means it is a contractor decision on where and how many, not the designer to dictate). I haven't ever seen the CMU, usually you just load up the rebar at the bridge.