Issue originally described here:
Sadret/openrct2-soft-guest-cap-calculator#4 (comment)

Reproduction Steps for the varying reported soft guest caps when harder guest generation is enabled:

• Start a new game of Alpine Adventures. The chairlift you start with contributes 55 to the soft guest count.
• Increase your loan to $20,000 and build the Woodchip wooden coaster prebuild in the park.
• Put Woodchip in Test Mode. After the stats are done calculating, a notification is sent saying the soft guest count is 265.
• Open Woodchip. You should receive a notification saying the soft guest count is 370.

I am able to reproduce this bug in both the current OpenRCT2 release (V0.4.1) and the latest develop build the OpenRCT2 installer gave me at the time of me writing this post.

I ran the scenario as described in the issue, but without opening the coaster. API reported SGC of 265. I then fast forwarded and easily reached 265 guests with no ads. So the problem is not with the plugin, but with the actual SGC. (Build: 564a902)