It's not "logical". It's metaphorical. Here's the etymology from the OED:
[OE. understondan, -standan (under-1 8 a), = OFris. understonda, MDa. understande, MSw. undi(r)standa, OIcel. (as a foreign word) undirstanda. Cf. MLG. understân to understand, to step under, MDutch onderstaen (Dutch -staan), MHG. understân, -stên (G. unterstehen), to take upon oneself, to venture, presume, etc.
With a different prefix, the same use of stand appears in OE. forstandan, OS. farstandan, OHG. far-, firstantan (firstân), and MHG. verstân, -stên (G. verstehen), MDutch verstaen (Dutch -staan). In the 15th and 16th cents. three forms of the past participle were current, viz. (a) the original understanden (also -stonden), in use till about 1550; (b) the reduced form of this, understande (-stonde), -stand (-stond), common till about 1575, and surviving into the 17th cent.; (c) the new form understanded (-stonded), very common from about 1530 to 1585. The occurrence of understanded in the Thirty-Nine Articles, xxxv, in the phrase `understanded of the people', has given rise to recent echoes of it, especially in journalistic use. The modern form understood came into use in the latter part of the 16th cent., and was usual by 1600. ]
So understand means to stand under (we would now say "stand behind" as a more common idiom), to know well, to have trust in, to have personal confidence in, etc. To say one understands a task, a trade, a fact, an idea, a job, the meaning of a word, or a person means all that. The personal aspect of the word is pre-eminent.