Vent below trap, see this way to often from out of the area plumbers on Residential & Commercial sites. Drives me nuts.
One wonders way they just didn't use a double san tee....A lot of extra work. (but maybe that's what was left on the truck?)
They could put an AAV on the fixture drain (connected to the vertical through a wye) and it would comply with the IPC.
Maybe the local inspector does not allow them?...I have seen that on more than one occasion....
Is there a state or municipality that does not allow double sanitary tees or is it an inspectors lack of understanding?