Thanks for writing back.
This screenshot with some extra debug information should help.
hidden link
The line '++ fitness, personal-care-services, teaching-training ++' on top shows that the review post "Personal Trainer – Shari Zisk" has a parent career post (Fitness Trainers and Aerobics Instructors: Salary, Job Description, How To Become One, and Quiz) with 3 tags: 'fitness, personal-care-services and teaching-training'.
This means that the parent view "(CAREER REVIEW) Related Posts" will get 4 career posts, where these tags are found.
And the next debug line shows that it brings these 4 career posts:
1. Fitness Trainers and Aerobics Instructors: Salary, Job Description, How To Become One, and Quiz (ID: 461708)
Tags: fitness, personal-care-services, teaching-training
2. What Do CrossFit Coaches Do (ID: 546562)
Tags: fitness
3. What Do Pilates Instructors Do (ID: 545938)
Tags: fitness
4. What Do Laundry and Dry-Cleaning Workers Do (ID: 462007)
Tags: lodging, personal-care-services
Now, the first 3 career posts have a related review post, so 1 review post from each of them is showing. But the 4th career post doesn't have any related review post, which is why the total review results in the final output are limited to 3 and not 4.
I think the main point that is causing confusion here is that you're expecting results from only 1 tag "fitness", but in this case, the parent career post (Fitness Trainers and Aerobics Instructors: Salary, Job Description, How To Become One, and Quiz) has two more tags (personal-care-services and teaching-training).
I hope this makes it more clear.