Paid for 3 year visa have 5 year visa - hey?

[pm07]

Hi All

Have a weird issue; I have a Tier 2 Visa via COS, issued March 2015. This is the old style visa before the biometric ones came in.
When I got the COS it was valid for 5 years. When I applied for the Visa I put in and paid for a 3 year visa. I was expecting to extend in 2018 but when the Visa arrived in the mail its expiry is 2020.

Can anyone interpret that?
Should I just assume I am good for the full 5 years or do you think I should still attempt to extend it in 2018?

[The Station Agent]

They've given you a visa valid to 2020 - so there's no need to try and extend in 2018. If you do they'll look at you like you're mad. Go with what they've given you.