Ken, possible answer to your question is my idea that the chart presents speed as Ground Speed. What it thus may show is the aircraft encountered significant winds shifted aloft.
Aircraft appears to have also made a significant course change, though if for weather avoidance or route requirements, I've no idea without a chart in front of me.
If the aircraft were actually below stall speed, the altitude would not have remained the same, since stall tends to yield a fall.
Presuming, as you note, a certain amount of accuracy in that table.