I just started working with new company and found they are using SFP-10G-LRM with single mode fiber. How could this be? I thought since the SFP-10G-LRM is Multimode, it can't work with single mode fiber. apparently I'm missing something here.
It can use multimode, but it doesn't have to:
The Cisco 10GBASE-LRM Module also supports link lengths of 300m on standard Single-Mode Fiber (SMF, G.652).
From the data sheet.
Generally speaking, any transceiver type that begins with "L" (for example: LR, LX, or LH) are designed to get their best range over SM fiber.
In some cases they may also be used on MM fiber, typically with reduced ranges. They may also require a mode conditioning patch cable to do so properly. You should check the respective data sheet for the transceiver to verify if it can be used on MM fiber and if it requires a mode conditioning patch cable.