Why are child-resistant packages sometimes easier to open by kids than adults?

Based on discussions with friends and colleagues there seem to be difficulties opening child-resistant packages in general. There are even stories of parents letting their kids (supervised) open child resistant-packages just because they couldn't open the package themselves.

This is not the intention of Medicines and Healthcare products Regulatory Agency which on the topic of Child resistant packaging:

Child resistant packaging is a useful deterrent in preventing accidental poisoning of young children. However, it is intended to be a last line of defence with safe and appropriate storage of medicines being the primary preventative measure in harm reduction.

A child resistant package is a package which is difficult for young children to open (or gain access to the contents) but which is not difficult for adults to use properly.

I guess that there is a difference between what authority intended and the outcome. The difficulty opening child resistant packages is also mentioned in the Wikipedia article Child-resistant packaging:

Child-resistant packaging can be a problem for some aged individuals or people with disabilities. Regulations require designs to be tested to verify that most adults can access the package. Some jurisdictions allow pharmacists to provide medications in non C-R packages when there are no children in the same house.

Which makes me wonder Why are child-resistant packages sometimes easier to open by kids than adults?