Yes chronos can be off. It is not uncommon to go to two or three different shops and get two or three different readings. That is also why you should shoot more than one arrow thru it. Shoot at least three and take an average. Most of them are made to check the speed of something small like a bullet, not an object that is 20 some inches long. And some are just cheap and don't work very well from what I understand. From what I have heard lighting plays a big part as well.

The worst are the techno hunt set ups from what I understand. They usually give readings different than a true chrono does. I don't remember if they are slower, or faster though. I think slower.

Bow scales are the same way. Getting an accurate reading off from a compound bow with a hanging scale is a little tricky. I bet you could have three people check a bow on the same scale and you might get three different readings. Not to mention if you are using a scale made to weigh things around a hundred lbs or more the scale will not be that accurate on the lower end. And some don't get calibrated very often.

If you are using a scale and a chrono to track the tune of your bow, like checking if from time to time to see if anything changes you should frequent the same shop and use the same equipment.

And I agree with the others, I don't think you are going to see 270 out of that bow with a 27 inch draw length and 63 lbs of draw weight. Unless you are shooting an extremely light arrow, which you are not. They tested the bow at 70 lbs, a 30 inch draw and a 350 grn arrow with nothing on the string. And they may have even fudged the numbers a bit to get 300 fps. You are losing around 20 fps alone just for your lower draw weight. Then add in your shorter draw length and heavier arrow. I'm actually surprised you are getting 260. If you shoot it thru a different chrono you may be dissapointed.

Paul