Initial calculation differs from on route calculation

🚧 THIS IS A TICKET MIGRATED FROM UPVOTY. The original ticket and comments will be available for a limited time over at:
https://abrp.upvoty.com/b/report-a-bug/initial-calculation-differs-from-on-route-calculation/
———————————————————————————
I started a small trip with 51%. ABRP said, I would reach the destination with 39%. I have no live connection (yet). During the trip, ABRP reduced the target estimation bit by bit to 32%. I wonder why the initial estimation is so different from the en route estimates. I didn’t drive fast: 50 and 80 as allowed and 110 where 120 are allowed.
(I reached my destination with 35%.)