That's exactly how it works. The example doesn't show more than one, but it will return multiple resolutions ranked with a confidence percent (the meaning of which depends on the method) as long as it's above a reasonable threshold.

Usually multiple results are a misdetection -- for example heavily compressed JPEGs will mess it up pretty badly:

    $ convert Lenna.png -quality 50 Lenna.jpg
    $ resdet Lenna.jpg
    given: 512x512
    best guess: 448x384
    all width        height
      448 (77.49%)     384 (90.19%)
      384 (66.99%)     448 (89.78%)
      320 (60.68%)     320 (72.54%)
      388 (55.92%)     256 (56.43%)
      445 (55.49%)   
      345 (55.47%)   
      242 (55.06%)   

(the block-based nature of JPEG compression/quantization creates a similar zeroing effect along every 1/8 of the spectrum)