I had the same doubt.
But what i think as an answer is the pressure inside balloon is higher because not the atmosphere pressure is same as pressurein balloon, but because effective force is same.
Let me explain
Suppose a normal balloon, now you blow it what actuallyyou are doing is creating high pressure in balloon so maintain an equilibrium balloon expands , Now pressure is decreasing but ...It will not decrease upto atmospheric pressure but a little higher ,Why??!!
Because I am neglecting something, as balloon is expanding it is gaining an elastic potential or tension along with a little bit decrease in inner pressure ( as area increase pressure decrease ) so elastic potential is like a ...just an example u r squeezing balloon with your hand, I know it is very rough example still helpful! And yah remember the tension of balloon skin is on balloon itself not on the air inside ( this was my misconception that tension of balloon wall is applied on air inside) .
So there are kinda more net OUTSIDE force i.e atmosphere pressure plus tension of balloon on itself , so to balance this net force the INNER pressure has to be high comparative to "Alone" atmospheric pressure.
Hope it helps.
Any expert here please check if I am correct or not but this is my understanding , correct if I am wrong