>what resolution are they measuring this frequency with

I've had a good look through her site but don't see anything regards resolution. Here's the link to the paper she's presenting http://www.animalvoice.com/catpur.htm - if you look 3/4 way down that page there are screenshots of recordings (don't know if it's relevant, it's over my head).

Here's the abstract of the paper:

A current hypothesis suggests the purr indicates contentment, however, cats purr
when they are severely injured or frightened. Forty-four felids were recorded
including cheetahs, ocelots, pumas, domestic cats, and servals. A Sony TCD-D8
Digital Audio Recorder (DAT) and Statham Radio microphones recorded the purrs.
FFTs and spectrographs were performed using National Instrument's Polynesia. An
accelerometer was also used to measure domestic cat purrs. Every felid in the
study generated strong frequencies between 25 and 150 Hz. Purr frequencies
correspond to vibrational/electrical frequencies used in treatment for bone
growth/fractures, pain, edema, muscle growth/strain, joint flexibility, dyspnea,
and wounds. Domestic cats, servals, ocelots, and pumas produce fundamental,
dominant, or strong frequencies at exactly 25 Hz and 50 Hz, the two low
frequencies that best promote bone growth/fracture healing [Chen et al., Zhong.
Wai Ke Za Zhi. 32, 217--219 (1994)]. These four species have a strong harmonic
exactly at, or within 2 Hz of 100 Hz, a frequency used therapeutically for pain,
edema, wounds, and dyspnea. An internal healing mechanism would be advantageous,
increasing recovery time and keeping muscles and bone strong when sedentary.
[Published with permission from the New Zealand Veterinary Journal; work
supported by Endevco.]