When you have tens of thousands of people each with their own story, you're bound to get some exceptions to the general rule.

The inmates' health would depend on when they arrived into the camp and from where; on the date of the arrest and thus their previous history as inmate; on their status in the camp (privileged prisoner? inmate doctor? functionary? secretary/typist?); on the ethnic/religious/political/social group one belonged to, as they were treated differently; on the nature of the liberated camp (some were worse).

With the exception of epidemics (like in Belsen), there was actually a good chance that quite a few inmates would look superficially normal. After all, many of the camps were slave labor camps and the inmates had to be able to work.

The phenomenon of the so-called "Musulmann" ("Muslims"), i. e. fully exhausted prisoners who could barely stand up and were often found in the "praying" pose, who basically gave up, was limited and most inmates tried to avoid that fate by any means necessary.

And normally the chance of seeing these exhausted people was actually low since they were usually regularly selected and murdered, albeit in the last couple of months these selections stopped being regular, thus some of them were liberated.

Also don't forget that e. g. when it comes to the Jewish prisoners, the selections of the newly arrived deportees in camps like Auschwitz meant that the sick, the disabled, the old would have been murdered even without becoming an inmate.

Of course, the exhausted and sick prisoners being unable to walk also meant that the inmates who greeted the liberators and can be seen in the first frames of the footage were bound to be the healthier ones.

Finally, "looking" healthy doesn't actually mean that much. That you're not a walking skeleton doesn't mean you're actually healthy.