You can if you just keep dragging a copy or mirroring a copy so all the steps are made from a copy of the first one you drew, then the dashed perimeter lines will all share an origin and they will line up correctly irregardless of the shape of each tread.
Yours don't because you were SLOPPY
when you created the steps and the edges don't precisely align. There are GAPS.
so in your screen shot we see lines that are parallel-ish
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"