How many strings?

This sentence, from Ann Hornaday's review of The Soloist, strikes me as very odd:

Downey plays Los Angeles Times columnist Steve Lopez, who, always looking to feed the beast that is a regular column, happens upon Nathaniel Ayers (Foxx) in downtown Los Angeles's Pershing Square, playing a dilapidated violin missing all but two strings.

I'm just not sure why she phrased it that. Why not "missing two strings"? Or, if she's afraid her readers won't know how many strings a violin has, "with only two strings"?

I don't know. It just strikes me as odd. "All but two" shouldn't be "two". A harp could be missing "all but two strings" - "all but" would be a significant number, 44 in fact. A sitar (with 21-23 strings) could be missing "all but two", or a dulcimer (15) - the "all but" would be much larger than the "two". A normal guitar, with 6 strings, would be oddly described that way; "with only two" would be more usual. And a violin, where "all but two" equals "two"? Just weird.