The field you want to sort on is a “string” (letters/text) and you’re trying to sort by numeric values. String sorting works more like an alphabetical list (i.e. replace the numbers with a/b/c/d and you’ll understand what it is doing). In order to make string sorting work as you expect, you need to pad with leading zeros.
For example, if you expect no more than 2 digits before and 1 digit after the period, then the numeric 1.1 should be 01.1 in your item description.