Companies release results after the close precisely so the market has time to read them without an intraday panic. The first minutes of the reaction are usually the headline numbers; the move often reverses once the call gives guidance, which is why the after-hours price at 16:05 and at 17:30 can differ enormously.
Spreads widen, depth is thin, and the price at 20:00 is a weak predictor of the next open, since overnight news and analyst-coverage revisions arrive in between.
Example: earnings beat and the stock trades $58 at 16:04, up from $52. At 17:10 management guides next quarter below consensus and it trades $47. The next day it opens $48.30.
Related: pre-market-session, extended-hours, earnings-report, guidance, closing-auction