tcping | Check if a desired port is reachable via TCP

What is the reason for this discrepancy?

tcpping just measures the time needed to establish the TCP connection. The connection establishment is usually completely done in the OS kernel, so there is not even a switch to user space involved.

Even some small data exchange at the application is significantly more expensive. First, the initial TCP handshake must be done. Usually only once the TCP handshake is done the client starts sending the payload, which then needs to be delivered to the other side, put into the sockets read buffer, schedule the user space application to run, read the data from the buffer in the application and process, create and deliver the response to the peers OS kernel, let the kernel deliver the response to the local system and lots of stuff here too until the local app finally gets the response and ends the timing of how long this takes.

Given that the time for the last one is that much off from the pure RTT I would assume though that the server system has either low performance or high load or that the application is written badly.