The range type takes input outside the range (and step) as valid. This seems different to the XLSForms specification. Only an additional constraint will make a (range) control working. So, it seems only a widget UI form, accepting any integer.
I have seen that range type was just added in last release:
Hello,
We meanwhile found out that range type is working in KoBo Collect, but NOT in online Preview/Enketo. (I will forward this to the differences list. http://support.kobotoolbox.org/collecting-data/overview-on-data-collection-tools) Those differences are a bit frustrating and might be better documented. We thought Preview is just a good way for testing a KoBo app.
Best regards