Which package versions are reported as available for your system can depend on your version of R. It can also depend on whether your R and RStudio are configured to install packages from CRAN, or from a locally managed repository. To figure out what's going on, it would also be helpful to see the output of running the following commands:
sessionInfo()
getOption("repos")
(please copy and paste the output here, then select it and click the little </> button at the top of the posting box to format it properly).
Also, where exactly are you seeing this? (In a console message? In the window that appears when you click Update in the RStudio Packages pane?)