Inboxing & SPAM is always a probability which is always less than 1 mathematically. I have studies SPAM filter in details.
So all your Mails cant always land in inbox always INBOX even though you hv highest reputation, a small portion will always land to spam and VICE Versa.
if you do a lot of spamming still you ll see some portion landed to inbox.
This ratio SPAM:INBOX depands on lot of factors like Engagement, Domain Reputation, IP reputation, Content, Link Reputation, assets link reputation users personal filters, Engagement Entropy, Domain Engaged unique users, report spams reports etc..
Unsubscribe link is a facility in Gmail which will help you in long run. for 2-3 tests in may land your mail in inbox if you remove it but do a test for 100 users+ to get ful report