Booking international flights using awards and cash

[redbatz]

Hi, last night I was trying to make a booking to Tokyo, partly to take advantage of DSC. So I saw classic awards Adel/haneda for around 31,000. so selected that - but when I went to select return flight - it had almost doubled!!?? - all the options. So I tried the other way. Selected return flight around $837 and then the flight over at 31,000 points. All good - it showed a total of just over $1000 and 31,000 points. But when I clicked over to continue - it suddenly changed to over 55,000 points as a "classic plus"!!? i tried over and over with many combos and different browsers etc - but still same. How does this work??