Determining how particular SwiftKey suggestions are generated
An answer to this question is very implementation specific, and can in fact only be answered for sure by a SwiftKey developer, eventually. In general, word suggestions are based on algorithms that are fed by n-gram dictionaries that predict possibilities for the next word you're likely to type.

See Wikipedia's n-gram entry to get more insight.

2-gram or 3-gram dictionaries are likely to be used with SwiftKey, amongst other techniques.

Regarding the strange sentence you auto-typed in: This reflects common use of probable 2/3-grams, so blame your fellow users on the poor spelling, it's just what they actually use.