How come AK74M is 3.4 kg(7.5 pounds) unloaded while its older version AK74 is only about 3.1 kg(6.8 pounds). What causes this weight difference exactly?

The one is constructed heavier.

Misread.

As fair as I know the real differences between the original AK-74 and a mil spec AK-74M is that the M has a fairly solid metal side scope rail riveted to the receiver, black composite furniture instead of wood, an integrated folding stock mechanism / rear trunnion (with associated hardware), an extra piece of semi oval steel plate welded under the pistol grip to reinforce the that area /  the rear of the receiver - apparently to protect the stamped receiver from damage when dropped when folded.  The furniture might be a wash but the other stuff is definitely adding weight.  On russian guns, I believe that they also may have stopped cutting the rings / grooves into the end of the gas piston (its just solid instead) so that could also add a little weight too.

As already mentioned, a lot of small changes that add up to more weight.

Even the 74M solid non ribbed dust cover is surprisingly more heavy than a ribbed 74 dust cover.

I find that all interesting...how in an effort to increase strength and improve the design overall, they wind up adding weight, which one was one of the things they were trying to reduce to begin with when they switched from the AKM to the AK-74. It parallels our M-16, which was really light to begin with, but efforts to improve it wound up adding weight.

In fact, they found the 74 was just a bit heavier then the AKM initially when they switched to it, mostly due to the smaller diameter barrel - smaller caliber = less steel removed from the bore = heavier barrel. That was one of the reasons for the horizontal grooves in the buttstock - to remove as much weight as possible, according to interviews with Kalashnikov himself. The lighter stock helped them get to the desired weight.

The weight reduction originally was the bare minimum for function, but for greater reliability and longevity, some things were beefed and even some steels were improved in strength over the older parts.

In developing the AKM, 0.5mm sheet was tried for the gas tube and top cover and found to be too flimsy.

The final design ended up using 0.7mm sheet metal for the gas tube and top cover.

With the AK74M the top cover was made even thicker.

The trunnion and bolt are made of stronger steels than used in the AKM, but were used in late AK74's.

The trigger parts are made larger in the AK74 and AK74M than in the AKM.

Part of that may just to be to use the axis pin sleeve inside them, but they are a bit beefer.

The scope rail is not only handy, but it also helps to stiffen the receiver.

The trunnion journal diameter is 1.0mm smaller than the AKM to allow a bit more material in the trunnion.

The Chinese took this step even farther, by compensating for their fairly new steel industry in the 1960's and reducing the trunnion journal diameter of the barrel even smaller and using a smaller diameter barrel pin (6.0mm vs. 7.0mm).