User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-26287497-20150518155942/@comment-7418318-20150518215826

Maybe due to being a mad-man, badly hurt and already being drawn back into his own time-line F-Rogue wasn't fully coherent and meant the "Gray killed Frosch" differently.

For example the one-year could have been meant not as the time of death, but as the start of the chain of events that would eventually lead to Frosch's death.

Imagine if Frosch and Gray started to spend more time together. Rogue grows jealous and after an argument storms off in anger.

While he's gone to cool off something kills Frosch and later Rogue blames Gray for it, because if it hadn't been for Gray, Rogue would have been together with Frosch and he believes he could have protected it.

I'm not sure if I'm joking or making a theory here... maybe both. ^^