Rollover max_size precision

I am seeing an interesting behavior on Rollover Index.
When "max_size" is given with little bit smaller than the current index size, sometimes (or some indices) the rollover succeeds sometime it fails.
For example, the current index size 2682mb and rollover "max_size" condition is given as 2500mb.

Is this an expected behavior? Any explanation?

I use this API to get the index current size:

GET /_cat/indices/myIndex*?h=index,pri.store.size&bytes=m

Curl command example:

POST /myIndex/_rollover
{
"conditions": {
"max_size": "2500mb"
},
"settings" :
{
"index.number_of_shards" : 5
}
}

Elasticsearch version: 6.8
I don't use any other condition.
I am adding more documents on indices so they are growing; I check the current size first before sending the Rollover API.

Thanks in advance,

Did little study on the ES source code and I see _cat and rollover API (condition check) use different stats - one is from lucene store directory and one is from lucene segment.
That might explain this issue b/c two stats may be different.
It would be good if someone know why they use different stats, if my observation is correct. And/or any suggested ES API to get stats from lucene segment (the stats that rollover uses).