I think he meant RG59. RG6 and RG59 are both 75 ohm coax cables. RG6 is typically used for connecting video sources because it's 75 ohm. Other coax's are designed for different applications and many have different characteristic impedances.
I think what the OP is asking though is all video coax created equal (all RG6). The ratio of the distance from the inner conducter to the outer conducter and the dieclectric in between determine the characteristic impedance of the coax. But some RG6 have less expensive connecters and sheaving, So even though they're
RG6, they aren't as good.