There is a tiny gap between each of the castsequence spells being queued which allows the lava burst request to be squeezed in when it is off cooldown. Castsequences do not queue the next spell in the sequence until the previous spell has finished casting. This requires the client to query the server to see if one spell has finished casting (approx 200ms based on my average latency) before sending the command to cast the next spell in the sequence. During this time, the command to cast lava burst is fired off and received before the next castsequence spell is.

This is the reason that the macro:

/castsequence lightning bolt, lightning bolt

used to produce inferior dps to

/cast lightning bolt

when spammed.

I am not sure if this is still the case with the new tuning that you can do with your UI to take into account average latency and if this can override the limitations of the castsequence process.

FWIW an identical system to /click can be engineered through sending multiple keybinds through a single keypress in Isboxer if you still prefer that system.