I think time is a huge factor with a project like this – the frequency of monitoring, the times of day, but also the duration of the monitoring period.
I’m guessing that most users here who have species counts of well over 1000 for their property didn’t get this number in the first year that they started monitoring it.
My experience with my admittedly tiny space is that it takes time to figure out how to most effectively observe the species that are present. What spaces are popular for which organisms, when to look for them, etc. And, often, there is also the time it takes to learn how to photograph them in such a way that an ID is possible.
I spent less time on my balcony this year compared to last (not working at home as much), but my hymenopteran species count is slightly higher simply because I upgraded my photography equipment, got more skilled at photographing quickly moving flying insects, and in some cases because I had learned what specific features were needed for ID and focused on this when photographing.
It also took me a while to realize that it was OK to handle organisms for the purpose of IDing them. This wasn’t about fear (I’ve caught or handled them plenty of times when I need to remove someone who accidently ventured into my apartment), but more about a feeling that they are just going about their business and it isn’t my place to disturb them.
For other organisms, it simply didn’t initially occur to me to look for them, or to record them, because their existence wasn’t something I had previously given much thought to.
Techniques like light attraction (moth sheets) or beating vegetation to capture small arthropods will also affect what one finds. I don’t think any such survey can ever provide an exhaustive account of the animal life forms present – there are too many factors. More intensive examination of the space will yield a higher percentage of the actual biodiversity, but there will always be organisms that get missed, for any number of reasons.
I suspect that for most projects like this, the species count is not so much a reflection of the relative biodiversity of a space as much as it is a reflection of the energy that the observer put into documenting it.
(I have a long way to go to reach 1000 species for my balcony: two years in, I’ve observed about a quarter of that – just over 250 wild organisms. But given that the space is less than 6 m2 I don’t think I’ve done too badly. I’ve barely done any nighttime observing, so I suspect there are still some significant untapped possibilities.)