_index.html and _index_high.html

Hello,

I am using W3TC 0.9.3 page_enhanced cache and occasionally I get two copies of a cached page and their related _gzip pages.
_index.html
_index_high.html

They appear to be identical. Why is the _index_high.html created sometimes?

Thanks,
Craig

https://www.ads-software.com/plugins/w3-total-cache/