It is not possible to have a single alarm with the severity updated each time a trap comes in with a differing varbind value.
With each trap coming in a new alarm will be created with the new severity based on the event discriminator in the varbind.
When using event discriminators you can create different alarms with differing severities based on varying varbind values in a trap.
So for example the same trap may be sent multiple times with different varbinds each time and based on this alarms with differing severities can be created.
This is discussed further in the Spectrum documentation under “Using Event Variable Discriminators to Generate Alarms”