Problem selecting the date with keyboard shorcuts

Open the datepicker. Then using crtl+arrow select the date and then press enter, the first time the date is selected, but if you repeat this process 3 or 4 times, the date selection fails and it keeps the old value. This always works using the mouse.

you can test it here http://jqueryui.com/demos/datepicker/

I am using firefox and iexplorer 7,8

thanks