Incorrect Inventory Adjustment Quantity when Using REST API

Greetings—

We have an external system that utilizes the REST API to adjust inventory levels in NetSuite. We have noticed, however, that sometimes the adjustQtyBy value that we supply in the request body is not the same value that NetSuite adjusts the amount by. What might cause this to happen?

Here is a recent example:

The system made a REST call like below:

{
"account": {
"id": <REMOVED>
},
"inventory": {
"items": [
{
"item": {
"id": <REMOVED>
},
"location": {
"id": 1
},
"adjustQtyBy": -6.0
}
]
},
"memo": "USED | 6 |"
}

This should have reduced the amount of the item in question by 6. However, on the NetSuite side we got this result instead: an increase