If your conversion rate exceeds 100% - i.e. 101%, 102% - it may be because certain browser types prevent us from recognizing Google customers returning to a specific webpage as a repeat customer. Therefore, each time the returning customer revisits your conversion page, we classify him/her as a new customer and record a conversion (instead of a transaction). In effect, your conversion rate may surpass 100% at times.
Another common reason is that the conversions is associated with the date of the click; not the actual conversion.
1. User's browser didn't accept the cookie.
2. User clicked ad on one computer, made purchase on a different computer.
3. Same user bought twice (so you would have 4 transactions but 3 conversions)
4. User's browser didn't fire the script or has scripts disabled.
5. Conversion happened post 30 days of the initial click.