Because if you choose a fresh 6-year H-1, then you are subject to cap which may affect the approval of I-129. Thus I believe your I-129 was approved assuming you were using the existing cap, which implies you were resuming the existing 6 years H-1.
When you apply the H-1 visa this April under the I-797 approval notice which was issued before your staying outside of 1 year, you will be having whatever left of your H-1 status.
If your H-1 is cap-exempt, it might be different.