Geoff is talking about the series support added to the firmware in 2.2.0. With that, the series name and number are stored separately in the database on the device.
I'm seeing the same thing. For the series numbers of 0.5, 1, 2, 2.4 and 5, the order was 1, 2, 5, 0.5, 2,4.
The number in the database is actually a string. Kobo will put other characters in there. The most common is to prefix the number with a hash symbol (#). But, one of the "A Song of Fire and Ice" books has something like "Part 1 of Book 2".
I tried adding the # in front of each number in the database. In this case I get 0.5, 1, 2.5, 2, 5.
I also tried adding the ".0" for the integers. That gives the correct sequence.