It isn't because of the Form 1040SR. Form 1040SR has all the same lines as "regular" 2019 Form 1040--it just has bigger print. The difference is in the tax forms themselves. They changed the form after that "postcard" size Form 1040 that we had for 2018 and came up with a different format for 2019. There were six possible "schedules" for 2018; for 2019 there are three possible schedules. And, yes, it is kind of hard to do a line by line comparison between 2018 an 2019.
Look at the forms:
2018 "postcard size---two pages printed on one sheet of paper
https://www.irs.gov/pub/irs-prior/f1040--2018.pdf
2019--Form 1040 and Form 1040SR the "lines" and schedules are the same for both
https://www.irs.gov/pub/irs-pdf/f1040.pdf
https://www.irs.gov/pub/irs-pdf/f1040s.pdf
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