This mismatch between open and reply can happen, and it's perfectly normal. This is due to the nature of inaccuracy inherent in how opens are tracked.
Any recipient can turn off "load images in email" feature on their email client, this can happen in gmail, outlook, iphone, android phone etc. it's a privacy setting. And if the tracking pixel is not loaded, it is impossible to tell if a recipient has opened an email.
The reply is a count of actual reply emails, that is certain to be accurate.