The issue first appears when rendering the plots. It does not appear to be a rounding error as changing the distance argument to other units affects the outcome.

library(ggplot2)
library(lemon)

data(sunspot.year)
sunspots <- data.frame(count=as.numeric(sunspot.year), year=seq.int(start(sunspot.year)[1], end(sunspot.year)[1]))
sunspots <- subset(sunspots, year > 1900)

ggplot(sunspo