Two questions re: bike computers...(sorry if this is a retread, I searched but did not find comparable threads...)

- Just installed a wired Cateye computer on a new bike. My existing bike has a wireless computer. Both of those read consistently higher in terms of speed and mileage accomplished compared to GPS readings done at the same time. I've double checked the setups to make sure wheel sizes, etc, are all correct. Why the discrepancy? Are bike computer companies conspiring to make purchasers feel better by inflating data?

- On my new (well, new to me) bike the rear wheel was evenly balanced. Now with the addition of the little magnet used for the speed sensor, the balance is off. This is not really noticeable (so far) except if I spin the wheel really quickly; one ride so far has not detected any issues. Any thoughts on ways to negate the imbalance, or is this something that, in real use, I typically won't feel?

TIA