It's an artifact from OD&D.
Rangers were originally introduced in The Strategic Review #2 in an article by Joe Fischer. That version of the Ranger cast both m-u and cleric spells. That article roughly coincided with the release of the Greyhawk Supp. The Greyhawk Supp. had the druid as an npc "monster" that cast both magic-user and cleric spells. So I'd surmise that the ranger casting both types of spells was inspired from the druid "monster" class - as in, once the ranger got high up enough in levels, it started to gain some "druidic" powers.
With Eldritch Wizardy about a year later, the druid was made a pc class and given its own spell list and the more familiar druidic powers. I'd guess, when OD&D was revised into AD&D, they just substituted the druid spells for the cleric without really considering why the ranger originally received both m-u and cleric spells.
If it bothers you, I'd say it would be a perfectly reasonable house rule to just double the number of druid spells the ranger gets.