There is no hard fast rule on what results in when email goes into spam or into the inbox. However, below are some insights or possible contributing factors.
1. Make sure to mark it as not spam. The more people that do that, the higher the likelihood of future email being delivered.
2. If it is your first broadcast email, you don't have a reputation yet, and that can lower the numbers. Even though the email is being sent out on a highly reputable email server, the email associated with the broadcast is not known. As you send out more broadcast emails, the combination of a reputable email server and reputable email will increase deliverability.
3. Many email servers will hold onto batches of emails from an unknown email source for a short time (typically up to 24 hours) before delivering them if they think there might be more coming. This is so they can determine if it is spam or not.
4. Some email servers route all group broadcasts into spam if the algorithms they use sees something in the content common to large broadcast emails regardless of the source.
5. Depending on the day and time an email is sent, it could take longer for it to "settle out" into the true numbers. Usually waiting 24 hours to make sure any bounces or server holds are resolved is a good idea before taking further action.
If after 24 hours you are not seeing an improvement (or a high bounce rate), you can request we resend the broadcast to only those emails that did not send a read response back. Most email servers send that response back to the sender, so this is a very useful way to increase deliverability for initial broadcast.