Literally, his sight had not yet fully recovered. The question is why not?
When there are two things, one represents a heavenly aspect and the other an earthly aspect of the same thing.
The man was healed spiritually and physically.
When his eyes/understanding was 'washed in the word' (of spit/water) he was healed spiritually first. He saw men as trees, which is the spiritual understanding of trees in sensus plenior.
De 20:19 When thou shalt besiege a city a long time, in making war against it to take it, thou shalt not destroy the trees thereof by forcing an axe against them: for thou mayest eat of them, and thou shalt not cut them down (for the tree of the field is man’s life) to employ them in the siege:
He saw men as trees near Bethsaida. Since he had been healed spiritually, Jesus commanded him not to return to the wicked city [1] the same as Lot was commanded not to look back at Sodom. Consider the phrase "Dogs return to the vomit".
The second healing was a healing of the flesh.
- Note: sometimes we say the tree is the cross without violating the rule that it must always be the same thing. It is just a different way of saying that the cross represents the 'life of Christ which was taken there' since the metaphor is not a simple word substitution, but a full bodied contextual meaning.
[1]
Mt 11:21 Woe unto thee, Chorazin! woe unto thee, Bethsaida! for if the mighty works, which were done in you, had been done in Tyre and Sidon, they would have repented long ago in sackcloth and ashes.