My question regards lengthening al medd al munfassil in the recitation of Hafs but by the way of At Tayyibah.

Question
Title: the medd al munfassil in Hafs

Question
Assalaamu aleikum wa rahmatu laahi wa barakaatuh
May Allah shower you with His Blessing and increase your balance of good deeds.

My question regards lengthening al medd al munfassil in the recitation of Hafs but by the way of At Tayyibah. I was listening to both sheikh Al Hussary and Mustafa Ismail (their murattal recitation)reciting suratul Qassas by the way of Hafs At Tayyibah.They were shortening the medd al munfassil to 2 counts until they reach the aayah 76: "Latanu.." when they made it 4 counts. I still do not understand very well this rule.

1. Why did they lengthened the medd to 4 counts instead of 2? I asked a similar question regarding aayah 29 of surah Al maaidah but the confusion is regarding the Hamzah. To me, when the hamzah is standing alone like in the word "samaa" then we lengthen 4 or 5. However when the hamzah is sitting on an alif, we only make it to 2 counts in Hafs At Tayyibah.

May Allah give you more knowledge.
Wa salaamu aleikum

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
And May Allah give you the same and more. 
The word: has a medd letter found by a hamzah in the same word.  This is what makes it .  This medd is always four or more counts for the recitation of Hafs.  The medd al- munfasil is when the medd letter is at at the end of a word and the hamzah is at the beginning of the next word.  The letter hamzah can take many shapes, or no shape at all.  It can sit on an alif, or a ya', or a wow.  These different shapes can happen in the middle or the word or at the end of the word.  .  We need to know where one word ends and where the next begins to know whether a hamzah after a medd letter is in the same word or at the beginning of the next word. 
Once we are proficient at determining whether the hamzah is at the end of a word or the beginning of the next word, then we will know whether to lengthen the medd letter preceding the hamzah four vowel counts or two when reading Hafs in one of the turuq at-Tayyibah which has two vowel counts for the jaa'iz al munfasil and four vowel counts for the waabij munfasil. 
May Allah increase all of us in beneficial knowledge and give us purity of intention always.
Wa assalaam alaikum wa rahmatullah.