Could be any one or a dozen of a hundred and one things,
@Blue2 .
Your bolus dose has clearly got to work before the 20g carbs turned into glucose and got in stream, and that could have happened for a whole lot of reasons. For example:
Too big a time gap between the bolus and eating - the insulin gets to work before the food is digested, so the glucose will be playing catch up with the insulin - every 5 mins added on to 20 mins after bolusing can make a serious difference.
Type of carb - sure, all carbs turn to glucose eventually, but they do so at very different rates and that makes a major difference to pre-bolus timing: piece of bread, I'll heavily pre-bolus for that cos that's fast, some pulses/beans, hardly any pre-bolus at all cos they're slow.
Were you talking and not chewing that much when eating? - you can eat exactly the same meal, if you bolt it down without chewing it that much, it's going to take longer for the partially chewed lumps to get digested and end up in stream if you've been chatting instead of chewing.
Maybe you were already on a downward slide going into the meal? Easy to see on cgm, not so easy on strips.
And then there's the other variables: wrong colour of socks!