"Chloe Steinbeck is a good example of a girl with decent 400m speed that moved up to the 800. She ran 56-mid in high school and got to 2:07 at Oregon."
Each athlete has many different variables, especially at the HS level, as proper coaching can be rare, also for many reasons.
For whatever reasons, many athletes have lost time/training that they can make-up later.
Had always planned on moving to 800m Senior year, but wanted to work on speed first.
At end of HS season this year, lost strength/speed and performance kept getting worse, with anemia symptoms.
Had blood work done, found out anemic, did not know any more so continued with summer track with worse symptoms and results.
Stopped training early-on and just went to a couple meets for fun. Stopped running sprints, as unable to perform, plus caused symptoms to worsen.
Tried 800m just for fun and ran couple times at evening meets surprisingly well considering the circumstances.
Had second round of blood testing when symptoms continued and switched Dr.s, then found had Mono, stopped all running and has since recovered. Early in May a team mate also had Mono, so it is assumed that is where she got it.
Here is a perfect example from the same time period as your example:
McKayla Fricker, Canby HS, Oregon----
http://www.athletic.net/TrackAndField/Athlete.aspx?AID=96777
400m PR thru 11th grade was 61.31
800m PR thru 11th grade was 2:17.79
400m PR Senior year/12th grade was 58.52
800m PR Senior year/12th grade was 2:11.71
She then went on to Pacific University, Washington----
http://www.athletic.net/TrackAndField/Athlete.aspx?AID=2700866
AND
2014 NCAA Division II 800m National Champion (2:06.19)
Current example is Alexa Efraimson, of Camas HS, Washington---
http://www.athletic.net/TrackAndField/Athlete.aspx?AID=3650447
Also a Senior in HS, her 400m PR is 57.74
At Brooks PR her 800m time was 2:03.26
I'm sure you heard, she is foregoing HS and Collegiate eligibility and signed with Nike.