Getting variance despite having only one replicate?

hopefully, a quick question. I am doing some stats work in R and I am getting error bars and variance for a treatment despite it only have one rep (the other 3 had no survivors in the experiment).

Can anyone explain how that would be possible? What is R actually giving me?

If it helps, here is my code for one of my graphs:

 ggplot(data = clammass, aes(x=TreatmentF, y=DryAvgMass)) +
  geom_point(stat = "summary")+
  stat_summary(geom="errorbar", fun.data = mean_se, width = .3)+
  scale_fill_brewer(palette = "Spectral")+
  theme_test(base_size = 16)