The simple explanation here is that in the first link, you are displaying image DESCRIPTION while in the two following links you are displaying image TITLE. Titles are bold, while descriptions are normal, so that we normally achieve something like this:
Some short title
Long description here lorem ipsum. Long description here lorem ipsum.
So the first question, should you not be consistent with using titles or descriptions or both? If you simply want to override and remove bold, you can add this to Settings > Custom > Custom CSS:
.images figcaption .title {
font-weight: normal;
}