area under normalized histogram is not 1 in plotly R

[ArunRegmi]

in https://plotly.com/r/histograms/, under 'Normalized Histogram' heading, histogram is generated as
library(plotly)
fig <- plot_ly(x = ~rnorm(50),
type = "histogram",
histnorm = "probability")

fig

The area of normalized histogram is as shown in the website is expected to be 1, but obtained close to 0.5.