In Japanese there is a pitch accent, some syllables have high and low pitch, this can make the same syllable sound different.
Also in Japanese there is Elision ( omission of one or more sounds such as a vowel, a consonant, or a whole syllable in a word or phrase, producing a result that is easier for the speaker to pronounce.) for instance especially with syllables ending in u is common, but equally including the u and putting emphasis on it.
The らりるれろ りゃ りゅ りょ syllables have the sounds r, l and d, kinda mixed together and it seems that sometimes that a different amount of emphasis can be put on the different sounds.
You will probably find that the usage depends on the speaker, where they are from etc. Try to listen to a wide variety of speakers and see how they say those syllables.