audience_size is bigger than total list size?



looks like I have a problem with list:
I am pretty sure that total amount of ids I’ve uploaded is about 5.5M. However sometime I’m getting results like this:

“data_type”: “tailored_audiences”,
“data”: {
“targetable”: true,
“name”: “list-dynamic-car”,
“targetable_types”: [
“audience_type”: “CRM”,
“id”: “z68p”,
“reasons_not_targetable”: [],
“list_type”: “TWITTER_ID”,
“created_at”: “2015-10-30T01:28:46Z”,
“updated_at”: “2015-12-14T09:18:51Z”,
“partner_source”: “OTHER”,
“deleted”: false,
“audience_size”: “7492440”
“request”: {
“params”: {
“tailored_audience_id”: “z68p”,
“account_id”: “18ce53vuu24”

I.e. looks like match rate (audience_size field = 7.5M) is bigger than actual list size (~5.5M). Not sure I understand how is it possible.
One side note - i am appending ids to this list time to time (and when I said above about approx list size i meant total, ie. I do track number of ids I’ve uploaded). Maybe this is a root cause since I do not see the same problem for ‘created-once’ lists.