Halliday & Matthiessen (1999: 318-9):
In general, wherever there is indeterminacy within a language, we may expect to find this reflected in typological variation. Let us briefly cite three further examples.(1) We saw that in the experiential grammar of the English clause there was a complementarity of perspective between the transitive and the ergative: processes may be construed either as 'one participant is doing something, which may or may not extend to another participant that it is being done to', or as 'one participant is involved in something, which may or may not be brought about by another participant that is the agent of it'. Probably all languages display this transitive/ergative complementarity in their transitivity systems; but at the same time it appears at different depths and in different proportions.(2) Secondly, we referred to projection as something that overlaps the 'boundary' between interpersonal and ideational metafunctional space; in English it is typically construed ideationally, though with a close relationship to the interpersonal systems of modality and mood. Other languages locate projection rather differently in relation to this boundary, sometimes foregrounding its interpersonal aspects, for example through a special category of 'reporting' mood.(3) Some process types tend to lie on the borderline between major categories, forming mixed and overlapping categories; typical of these are the behavioural and existential processes in English. It is likely that equivalent types of process will be liable to greater typological variation than those that fall squarely within the core categories of material, mental, and relational. …
The phenomena of human experience are held in tension by so many intersecting analogical lines that, while all of us have the same brains and live on the surface of the same planet, such diverse ways of semiotic mapping are not only possible but inevitable.