R and Ř are different letters, as opposed to modified base letters(?) like German umlauts with ö vs o
From Czech language on Wikipedia (my bold)
The acute accent (čárka) letters (Á, É, Í, Ó, Ú, Ý) and the kroužek letter Ů all indicate long vowels. They have the same alphabetical ordering as their non-diacritic counterparts. ...The háček (ˇ) indicates historical palatalization of the base letter. The letters Č, Ř, Š, and Ž currently represent postalveolar consonants and are ordered behind their corresponding base letters; while Ď, Ň, Ť represent palatal consonants and have the same alphabetical ordering as their non-diacritic counterparts.
A test with Ö and O in Swedish (different letters) and German (modified)
WHERE N'Ö' COLLATE Finnish_Swedish_100_CI_AI = N'O'
WHERE N'Ö' COLLATE Latin1_General_CI_AI = N'O'