If your blog has, say, 100 followers (for easy numbers), then when you make a post--any post--the MAXIMUM number of people who are *guaranteed* to see that post on their dashboard is 100 people if you don’t tag anything (for easy numbers).
Unless your blog is hyper-specific, it isn’t likely that all 100 people are actually interested in that content. They might follow you only for your cute animal posts, not your flower photography posts. So they skip it. Maybe that leaves 75.
Not all of your followers are going to be online at the same time, or even on that day, so we’re probably down to 50.
And, don’t forget the bots and porn phishing blogs. Let’s make it 40.
If all 40 of those people just LIKE the post, you’ve got 40 notes. It goes no further. That’s it.
If all 40 of those people REBLOG the post, you’ve got 40 notes, BUT they’ve put it on their dashboard for all of their followers to see, so if half of those reblogs attract the attention of at least one person (REBLOG or LIKE), you’ve got 60 notes. Then, maybe a half of those were reblogs, and half of those attracted the attention of at least one person, so you’ve got 65 notes.
If half of those original 40 reblogs get 2 notes instead of one, you’ve got 80, and if half of those were reblogs and half of them got someone’s attention, that’s 90.
INSTAGRAM is designed to passively let you like whatever you want. It doesn’t get shown to your followers, it doesn’t show up on your page. TWITTER is designed so that the likes of the people you follow get shown to you. TUMBLR treats likes similarly to Instagram. If I like something and do not reblog it, my followers WILL NOT see it.
The difference between TUMBLR and INSTAGRAM is that Instagram does not give you the option to share what you like with your followers. Tumblr DESPERATELY WANTS you to share what you enjoy with your followers. THAT IS WHAT MAKES UP THE COMMUNITIES ON HERE.
Communities on this website do not grow and thrive if everyone is just liking things they see without sharing it to their followers.