Blurrier subtitles and GUI in UHD resolution

Hello!
(Sorry for my English)
I am looking for a solution for the following.
I bought a UHD TV. I noticed that on the UHD TV, the Kodi GUI and the movie subtitles (only the movie subtitles, not the movie itself) are sharper in FullHD resolution than in UHD resolution. Why is this? The TV has the same picture settings for both resolutions. (System CoreElec Omega)
Thanks in advance for your answers!