Isn't the limit exact 21 Million BTC?
No.
Anyone that told you that has misled you.
The correct phrase is "There will never be MORE THAN 21 million."
Saying, "There will be exactly 21 million" or "Bitcoin is limited to 21 million" is inaccurate.
Why minus few satoshis?
The original designed maximum possible bitcoins to be created is:
20,999,999.9769
That's 2,310,000 satoshis LESS than 21 million. (which in my opinion is more than just "a few").
Then, due to a bug in some miners early on, some of those 20,999,999.9769 bitcoins were not mined (and therefore can never be mined). Which reduces the total bitcoins that will ever be mined even more.