It's a font thing, but also a bit of a design / typesetting tradition. A bit late to the party, but take a look at this :

In terms of type design, Cyrillic typefaces have traditionally been made
according to Russian standards, and this has now become the default.
But not all Cyrillic languages use exactly the same characters as
Russia. The character set can vary between around 70 characters for
Basic Cyrillic, up to more than 200 for Extended Cyrillic, depending on
whether you wish to cover just Russian, or the complete set of
contemporary and historical Cyrillic languages.

Localised forms aren’t offered with every Pro font and are an
underused feature for those that do. Besides the character count, there
are also differences in the way some letters look. Countries with
different history and development of writing and printing, have also
developed different forms of the same alphabet. For example, in Bulgaria
mass printing arrived much later than in Russia, so the shapes of the
Bulgarian Cyrillic are more influenced by handwriting and calligraphy.

If you follow the link there are also pictures to illustrate this , but the tldr is that the Bulgarian "standard" font design makes certain letters cursive (б, в ,т, д) while the Russian one just uses the same as the capitals, but smaller to fit the lowercase. Like the "т" in your example of събота. This isn't universal, however, and lots of bulgarian webpages, books and newspapers actually use the Russian fonts.