Somebody may come along later with better sources, but I think the reality here is that acronyms like this are often pronounced with fairly even stress, so marking a "correct" stress pattern is difficult.
In early 20th Century "BBC English", a radio announcer would have paused between each letter, rather than treating it as a word: "This is the Bee Bee See Home Service". Spoken quickly, that might lend a slightly stronger stress on the first B. On the other hand, if you do run it together as though it was a word "beebeesee", the natural stress would fall on the last syllable, and therefore the C.
DVD is a much more recent word, and more often spoken as though it was a word, "deevidee" or "deeveedee". The main stress is definitely on the last syllable, but there is probably some variation in how the rest is pronounced. The difference in secondary stress marker on the middle syllable is probably referring to speakers either pronouncing a full "ee" sound, or weakening it to a vowel more like the "i" in "hit".