Thanks for leaving the question.
That's a great point out!
To be honest, I pointed the same thing to my co-worker in KoreanClass101.com.
"Haha~~!" So I really do understand what you are saying.
My "personal" answer is here...
First, you may say both "ㅂ (b/p) and ㅁ (m).
As many languages have the same issue, Korean is one of them.
Think about "what's up!" Some people say, "wha seop!"
some people simply say "seop"...
The point is, in many languages, writing and speaking are different.
In writing, "잘 먹겠습니다" is correct.
in Speaking, both 'jal meok-get-seup-nida' and 'Jal meok-get-seum-ni-da' are correct.
Especially, when you speak fast in Korean, you will find so many changes in sound.
Personally, if you want to learn Korean academically, it's better to write and say, "jal meok get seupnida"; however, if you want to learn korean for pleasure, it's good to say,"jal meok get seum nida".
There are so many rules for the changes in Korean syllables in speaking.
My advice to you is you learn both - writing and speaking together.
Let's go back to your question.
why do we say, "seum" instead of "seup"? Because we tend to speak Korean faster.
Is it wrong to say "seup" instead of "seum"? No! No one makes problem with that (I think...)
I could create lessons for pronunciation in Hangeul, which will cover so many changes in sound and rules for pronouncing words. But, I don't have a plan for that yet.
Again, thanks for your question.
I hope my answer will help you understand better in Korean.
cheers,
Tim
ps. "Thanks George!"