Initial light primer strikes - something not right with either how the hammer spring was engaging the hammer, or something off with the hammer not fully engaging with the firing pin on those initial rounds.
It is plausible (and probably more likely) that if your chamber is rough or tight, that on some initial early round that you had firing pin protrusion or a "blown primer", and that blew a bit of primer material back through the firing pin opening in the bolt, and that it lodged somewhere within the firing pin channel just enough to foul good solid hits on the harder primer'd stuff and not rounds with lighter primer cups.
At some point in your effort to diagnose the light primer strikes or through regular firing, that little piece of deleterious material worked its way out and you were then seeing solid hits back on the harder primers.
edited for spelling.