Highs are usually recorded then because that is when they usually occur.
However, that doesn't mean they are ONLY recorded during that time.
There have been many days where the nighttime temps are warmer than daytime (usually happens in winter where a warm system moves in after a cold snap) .
So a high temp can be recorded at 12:01 AM, or 12:01 PM.
Hence, the forecasted highs and lows for tomorrow are probably accurate.