Today, I've tried sharing photos using NFC for the first time.

Found it very convenient compared to bluetooth sharing.

I just have a question. I thought NFC has a limited sharing range/distance, meaning that the two devices have to be within a close range of approximately 10 cm. This has been my understanding so far.

Out of curiosity, I tried increasing the distance, and found no problem within 4-5m.
I did turned off wifi and 3g as well. 

How is this possible? Am I missing something here?

Of course, when you pair the devices, they have to be side by side but once it's paired, I found that the devices can be apart 4-5 m and still the files will transfer without any problem.

Can someone explain this to me?

Just to make myself clear, I did photo sharing through NFC, not through wifi direct or bluetooth.
It was between nexus 5 and galaxy s5.

Cheers