Yes, I agree with Mandy... using Visitor based segments is particularly impactful, since it will look at everything the user did within the panel range.
For instance, let's say you have a segment that looks at:
Visitor [
Page equals contest
]
And you want to see a conversion rate from viewing the contest page, to signing up for a newsletter.
Visit 1 (Jan) - viewed the contest page
...
Visit 8 (Apr) - signed up for a newsletter
If you look only at April, this user wouldn't be included, they didn't look at the contest page in April, but if you change your range to "This Year", the action they took in January makes the user eligible, and the Newsletter Sign Up counts in your conversions.. making April inflated.
You also have to keep in mind, that if you are trying to use the Unique Visitor metric, you don't need to use a Visitor scoped segment... this is a common misconception.
If you share with us what you are trying to use, and what segments are being applied, we should be able to help you.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.