the problem is that instead of averaging the measurements when you zoom out, it simply takes the nearest observation. Basically it shows you the temperature exactly at that time (3pm in your pictures), not the temperature at 3.30pm.
Not all data is stored at the servers. It’s explained here: Data archive buckets explained
It is my long standing grieve that the graph only shows 32 points at a time. All the modern screens can show way more data points. And for a big part of your graph it did store all the details and just should show them and let us zoom and scroll freely.