Excellent question, fellow language learner! “Testimonials” are reviews (up to three) chosen by the teacher. This lets the teacher highlight the good reviews, and perhaps divert attention from some of the other (more representative) reviews. It seems to me that any teacher can manage to get three good reviews during their career, so in my opinion, the “testimonials” are the least useful information for prospective students who are looking to select a teacher. Personally as a student, I always ignore the “testimonials” (and go directly to the “non-testimonial” reviews) when looking for my teachers. It's also a good idea to divide the number of lessons by the number of students and then subtract one (the first lesson is not a repeat customer) to see how much repeat business they're doing. When the “testimonials” first came out, I wrote management and told them of my concerns, but of course they ignored me. Naturally, if you can’t beat’em, joing’em, so after about a year, I reluctantly chose three reviews myself. 

こんにちは、リコさん。

Highlighted testimonial is teacher chose them to make them stand out. :)