How does value decompression work for Facebook's Gorilla in the case where count of leading zeroes is not stored
I found my answer after reading the reference implementation: https://github.com/facebookarchive/beringei/blob/92784ec6e22572f28500c76b669276007635c875/beringei/lib/TimeSeriesStream.cpp 

The use of the term "meaningful bits" in the paper is ambiguous. I thought the sentence "use that information for the block position and just store the meaningful XORed value." means to store the bits with all the trailing and leading zeroes stripped because "meaningful XORed value" is a value with no leading and trailing zero. But, according to reference implementation you just strip off same amount of leading and trailing zeroes as previous value delta; the meaningful bits in this case may still contain some leading and trailing zeroes. 

So, for the example in the question, we should store 0ff0 as meaningful bits instead of just ff.