I agree this seems non-intuitive until you understand the discharge profile of the lithium batteries. It is nearly flat for a long time and then drops extremely quickly - the voltage output “falls off a cliff”. The point where the SKY stops operating is when enough of the batteries drop off the cliff, but they don’t all hit the cliff at the same point, thanks to normal variability in the batteries. So, your 0.5V batteries have already fallen off the cliff and the batteries that are just under 1.5V are likely to be near the edge of the cliff:
By contrast, alkaline batteries have a much smoother discharge curve and drop below 1.5V in the “flat” section, so your “dead” batteries would be more likely to have similar voltage levels:
(graphs are from http://data.energizer.com/pdfs/l91.pdf and http://data.energizer.com/pdfs/e91.pdf)