In referring to the same source (transit55) for some more information, I have discovered that this bus went into service on this block of work at 09:12am. Given that information, and given that it was not dispatched from the beginning of the service day, there are a few things, one of which likely happened, resulting in this out of the ordinary spotting.
1. The regular bus broke down or had a mechanical issue and this bus was used as a road change off to replace the bad order bus. The fact that it was an Articulated Bus for a 40 foot bus or a Spring Gardens Bus for a Victoria Park Bus is irrelevant to a Bus Controller if it means preventing or reducing loss of service. A conventional bus is a conventional bus.
2. The regular bus broke down or had a mechanical issue and for what ever reason, Victoria Park Garage was unable to change off, resulting in a Spring Gardens bus being used.
3. Something happened to the Operator, such as illness, where they were unable to carry on. Given that Spring Gardens is now the 24/7 garage, it is not uncommon for them to have a plethora of standby Operators on hand at all times of the day in the event that this happens. If this is the case, they likely sent an operator out with a new bus, as opposed to the operator finding their way to the original bus. This allows controllers to send the new operator wherever they need to get back in service ASAP.
4. Given that a number of the floater buses for the MAX line come from Spring Gardens, and given there is no strict direction that they can only be used for MAX, there is a possibility that one of these buses was used to replace service on the Route 301, for what ever reason (accident/incident, illness, breakdown, etc)
Given the information above, if in fact one of those scenarios is correct, which i don't doubt for a second, that speaks to the spotting of 8114 on the Route 156 and shows the irrelevance to the question of "Why was a Nova, with the fewest seats of any 40 foot bus was used?" With that being said, There are some logistical challenges with such a change off. There are still quite a few Operators that are not trained on Nova buses, as well as quite a few operators not trained on Articulated buses. There is the risk that a relief operator may not be trained to drive the Nova.
Due to the late finish, this bus would have been dispatched, the next day, strategically, out of Victoria Park Garage on a Victoria Park Run that ends early in the evening, if not after the AM rush hour, which it was. This allows the bus to be ferried to the proper facility. I would not be surprised if this bus is back in service at its prescribed facility the next time it is on a regular block of work.