When Ivy syncs to QBO, there are a ton of data points on Ivy that have to be stitched to their counterparts on QBO. Understanding this helps bring clarity to how the sync between Ivy and QBO works. 

An Invoice isn't just a single data point, right? It is really a combination of many different elements. An Invoice has a customer, items (and those items have categories), tax rates, etc... These elements need to exist on QB in order to put them together on an invoice.  For example, you can't add an item to an invoice until you have added that item to your products list.
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So, when Ivy initiates the creation of an invoice in QBO, Ivy must also verify that all the invoice elements exist. More specifically, Ivy first must verify the customer, items, categories, tax rate, etc. exist within your QB account. Once Ivy identifies these elements, the invoice can be populated accordingly in QB.

For example, let's take an item: Blue Sofa.  Ivy essentially asks, "Hey Quickbooks, do you have an item in your products list called, Blue Sofa?"

If Quickbooks answers yes, then Ivy will use the product that already exists in your QB account. 

If the answer is no, Ivy will create a new product in QBO called Blue Sofa. 

.....In Ivy, the Blue Sofa has other attributes, too! It is tagged as category furniture! Just like Ivy did with the item title (Blue Sofa), Ivy will now ask QB another question:
​"Hey Quickbooks, do you have a category called furniture?"
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If yes, Ivy will use the category furniture that already exists in your QB account.

If no, Ivy will create furniture as a new category in your QB account.

This occurs for all relevant elements (and their attributes) so all the components are available for the creation in QB.

In summary, once Ivy knows that all of the elements exist on QBO (whether they were created by you in QB, or Ivy initiated the creation), only then Ivy initiates the creation & successful sync of the invoice.  Ultimately, making a twin of the Ivy invoice in your QB account.
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(Of course, these computers are pretty smart, so the above happens in just a couple seconds.) 

Here is video to go into more detail: