Would be interesting to know how long their viewport is, this would help know what google would see when crawling.
I noticed this effect on an eCommerce site I wrote where the top x products would get indexed but as they were also within the scroll area it was not clear where the cut off is.
I must run some tests to determine how far down the page they actually read.
Bit of a clue here by John Mueller of Google via twitter.
Googlebot renders with a very tall viewport, which skews some CSS (often images). Try in Chrome dev-tools, eg 9000px high viewport.