Student faculty ratios can be reported using "full time equivalent" faculty. Meaning, your class of less than 20 students might not have a more available professor, because they're actually also teaching at 2 other universities in the same system to make ends meet. It's not the ratio to the number of tenured faculty, adjuncts count.
For class size, you can have dozens of small classes nobody takes, and have giant lectures for the core intro classes everyone takes. Then the average class size of the classes offered is quite small, but the average class size of the classes you take is quite large.