Humm, we DO call them Sundays IN or OF Lent, not Sundays surrounding Lent.
In days when fasting was a regular feature of weekdays during Lent, it’s true that Sundays were not included in the Lenten fast. I think that’s the source of the thought that our personal penances do not apply to Sundays.
If I give up something, or DO something (and one can hardly do something extra without giving up something such as time), and I say this is for the Lenten season, then it seems to me that Sundays in/of Lent are included.
Just a thought…