I opted for 40 Mbps speed but Ookla showing 33.42 or sometimes even less. I use a Netgear WNR614 N300 wifi router. What could be the possible reasons? I am using Windows 10.
There are lots of possible reasons, however thr most likely are your ISP (or uour ISP data pool for your class of service) is congested and is a bottleneck or other computers on your network are active and using data.
Its also possible, but less likely that your WIFI is causing the issue, pissibly due to interference. You csn rule that out by plugging your PC directly into the router using Ethernet.
I point out that your result is not actually that bad.