'Wuthering Heights' - Why does Kate Bush sing "Heathcliff" in that way?

A had an interesting conversation about music with a lady in my office the other day and I've been thinking about it ever since. It all started with her asking me whether I'd read Wuthering Heights. I replied "Yes, which is more than can be said for Kate Bush when she wrote her hit song." This led to the inevitable comical renditions of the song and then onto a conversation about Kate Bush's pronunciation of "Heathcliff" or as she sings it, "Heath-Cleef".

The lady said that when she used to take classical singing lessons her teacher would encourage her to sing her i's so they sounded more like ee's. She said the i in Heathcliff is sung as more of an ee because it sounds better, i's are better avoided if possible because they don't sound as good when sung. This does sort of make sense and is the only plausable explanation I've heard so far for Kate's pronunciation but I can't find anything about it anywhere on the internet. Can anyone verify this?

In the meantime, here's Kate in that red dress singing her 1978 classic.