Google is getting confused because while the page is ISO-8859-1, some content is loaded into the page in UTF-8. This causes Googlebot to have to re-encode the content page content as UTF-8 so that it can process it. Something is going wrong during that process and characters are getting mangled.
Ideally Google would be able to deal with this situation without getting the characters garbled. I contacted Google about this and a bug has been filed on their end. However, some other sites that are getting re-encoded are working. Whatever is happening with your site isn't affecting a lot of other sites, so it may be a lower priority fix for Google.
In general, there is no good reason to use ISO-8859-1 these days. That character set only support 256 characters. UTF-8 doesn't make the page size significantly larger and it supports all unicode characters:
- The extra French characters Œ, œ, and Ÿ
- The Euro sign (€), ellipses (…), non-breaking space ( )
- Fun characters like arrows and emoji
Using UTF-8 allows you support user generated content from any language. At the very least, it allows users names to be written correctly, no matter their national origin.