Thanks for the observations!
What you are observing here is a slight variance in how often these attacks could occur. In Original WoW you could also experience both shorter and longer windows between hateful strikes as well. This slight variance still exists in the WoW Classic just as it did in Original WoW but is somewhat less pronounced, and the overall average damage output is extremely consistent between the reference client and WoW Classic. As a result this is its not something that we are looking to make changes to at this time.
I’m going to go ahead and close this this thread now as we do not currently consider this to be a bug. As always, thanks for the report!