The drop in Jan 2017 was due to Hurricane Harvey which disrupted foreign crude deliveries. https://www.mercurynews.com/2017/08/31/us-taps-strategic-oil...

You are right that it wasn't 77M barrels in 2020 - my bad. I didn't realise that Democrats had blocked funding for that proposed initiative of SPR increase. The Trump administration worked around this by reducing this to ~30 million barrels and awarding storage contracts. This appears to match your figures.

https://www.bloomberg.com/news/articles/2021-06-25/trump-s-t...

https://www.bloomberg.com/news/articles/2020-03-25/funding-t...