If the antenna is mounted on a horizontal arm attached to a vertical mast you are moving the antenna around in a circle.
The higher the frequency shorter the wavelength gets so the greater the distance the antenna appears to move in relation to wavelength. You only need to move 1/4 wavelength to go from a peak to a null. 1/4 wavelength is only 20" at VHF high band down to 3" at 900 MHz so you don't need to move the antenna that far to see effects on weaker signals. The same thing happens when you move a hand held around or drive a vehicle.
If you kept the antenna in the center of the circle and rotated the mast around the antenna you would not notice as much difference.
A certain manufacturer advertizes that their cheap omni scanner antenna acts like a yagi when you rotate it around the mast like that.
I don't buy it. I would like to see somebody test it properly on an antenna range or at least with antenna modeling software to see how much directivity it really has.