rgdal project; inverse does not return original values

I am confused about this behavior of rgdal

require(rgdal)
# make a matrix of coordinates
xygeo <- rbind(c(-180, 0), c(180,0), c(0,0), c(0,20), c(0,90))
# project to sinusoidial
xysin <- project(xygeo, "+proj=sinu", inv=FALSE)
# project back to longlat
xyinv <- project(xysin, "+proj=longlat", inv=TRUE)
xygeo

[,1] [,2]
[1,] -180    0
[2,]  180    0
[3,]    0    0
[4,]    0   20
[5,]    0   90

xyinv

[,1]      [,2]
[1,]  180         0
[2,] -180         0
[3,]    0         0
[4,]    0 126759249
[5,]    0 573070423

Why are rows 4 and 5 different?


An illustration of the same problem, using 'raster'

library(raster)
r <- raster(nrow=36, ncol=18,  projs= "+proj=longlat")
xy <- xyFromCell(r,1:ncell(r))
par(mfrow=c(1, 3))
plot(xy) # ok
xysin <- project(xy, "+proj=sinu", inv=FALSE)
plot(xysin)   # nice
xyinv <- project(xysin, "+proj=longlat", inv=TRUE)
plot(xyinv)  # ???

Robert

sessionInfo()

R version 2.9.0 (2009-04-17)
i386-pc-mingw32

locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
States.1252;LC_MONETARY=English_United
States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] rgdal_0.6-8 sp_0.9-34

loaded via a namespace (and not attached):
[1] grid_2.9.0      lattice_0.17-22