While eHealth shows strictly the sum of ifInDiscards and ifOutDiscards, the value of Discard Rate shown in the Spectrum interface Performance tab includes ifInDiscards, ifOutDiscards and ifInUnknownProtos.
The value of Discard Rate shown in eHealth is more useful in the context of quantifying how oversubscribed a port might be - its buffer can't keep up with all of the data it's passing, so it's dropping some packets.
In contrast, the value Spectrum shows includes ifInDiscards and ifOutDiscards as well as ifInUnknownProtos. ifInUnknownProtos represents the number of packets that were discarded because the device had no knowledge of the protocol. The Spectrum value would therefore be more useful in the context of troubleshooting why certain packets didn't make it through. It takes into account all of the possible ways (oversubscription and/or unknown protocol) packets might have been dropped.
Here are some relevant explanations from the Cisco online document "Troubleshooting Switch Port and Interface Problems":
"A port buffers the packets to the Tx buffer when the rate of traffic switched to the port is high and it cannot handle the amount of traffic. The port starts to drop the packets when the Tx buffer is full and thus increases the underruns and the output buffer failure counters. The increase in the output buffer failure counters can be a sign that the ports are run at an inferior speed and/or duplex, or there is too much traffic that goes through the port. As an example, consider a scenario where a 1gig multicast stream is forwarded to 24 100 Mbps ports. If an egress interface is over-subscribed, it is normal to see output buffer failures that increment along with Out-Discards."
"Unknown protocol drops are normally dropped because the interface where these packets are received is not configured for this type of protocol, or it can be any protocol that the router does not recognize. For example, if you have two routers connected and you disable CDP on one router interface, this results in unknown protocol drops on that interface. The CDP packets are no longer recognized, and they are dropped."