First of all, at what temp did you correct. While there are estimators for this correction, anything over 100 degrees is really less than optimal and the readings can be quite off.
That said, most pre-boil OG readings IME are about 8-10 points from the post boil so you can assume with a normal boil over 60 minutes your OG will increase by that amount.
So if you miss your pre-boil you can opt to add DME, boil harder or longer or dilute depending upon which way you miss.
You added DME to raise your gravity but then boiled too hard and experienced more boil off, hence the higher OG.
Boil off can vary depending on how hard you boil or ambient emperor ire and humidity. Cold dry air=more, hot humid =less
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