I am new to postgis, so please excuse what may be a very misguided question. I'm trying to reproject coordinates into a standard datum, but in some cases I don't think I'm getting a realistic result. The most obvious is from

The coordinates you provided are most likely already in WGS84 (not in EPSG:4813), and therefore do not need to be transformed:

SELECT ST_SetSrid(ST_MakePoint(107.8, -6.23),4326);