How on earth did mikrotik manage 6.5 dbi gain on such a short antenna?
Whether or not an antenna is short for a meter-long antenna can be determined based on the wavelength.
Do you know the wavelengths for ~824-960 MHz? 
I use other brand shorter one with 10dB
You linked to the old, discontinued model that is 950 mm i.e. almost one meter, not so short, but of course it depends on what you compare it against.
The new one is this:
https://mikrotik.com/product/868_omni_antenna
and it is only 500 mm long.
Same 6.5 dB.
Good catch, yes I meant the 500mm one.
Plenty of false claims out there from generic products.
Yes, do you?
A quarter wavelength antenna would only need to be around 100 mm long. With a length of 500 mm, the antenna is 1 to 1.5 wavelengths in length. 6.5 dB gain seems reasonable.
This one (example, first result I had searching for 824-960 MHz) is 6 dB and 600 mm:
https://www.pasternack.com/6-dbi-omni-antenna-824-960-mhz-n-type-female-1.5-1-typ-vswr-pe51om1014-p.aspx?
6.5 dB in 500 mm doesn't seem impossible.
We tested both the old, long one and the new short one and the long one performs better in all our tests. The only problem is they are very fragile and constantly break in high winds.
The trick is actually the used unit: dBi is in relation to an isotropic antenna. That´s a SINGLE POINT in space. So for a lambda/2 dipole antenna, which is the most simple antenna design, you already get 2.15dBi boost. This is an age old sales trick of every antenna manufacturer. For 900MHz you have 30cm Wavelength, so yes it´s realistic. You have to consider, this antenna wont radiate much in the upward direction.