Why docCount differs on primary and replicas of the same shard

[krlm]

I'm trying to figure out why executing the same query multiple times gives different results when there's no background indexing involved. What I've found so far (using "explain: true") is that docCount returns different values for primary and replicas of the same shard, where I'd expect that replica is a bit to bit copy of primary shard.

example: first search returns following data (just first

 "_shard": "[unittypes][0]",
 "_node": "-o4ti0RvSZePKozz5_cxqA",

 ...
	{
		"value": 11325,
		"description": "docCount",
		"details": []
	}

second attempt returns results in slightly different score results (even for the same document)

"_shard": "[unittypes][0]",
"_node": "Hgn_AK63Th65mDr9S5-ReQ"
...	
	{
		"value": 11317,
		"description": "docCount",
		"details": []
	}

I've checked shards stats and it seems at least here doc counts are OK.

unittypes 1 p STARTED 233204 124.9mb 10.0.0.5 estest0
unittypes 1 r STARTED 233204 125.1mb 10.0.0.6 estest1
unittypes 2 r STARTED  92044  76.2mb 10.0.0.4 estest2
unittypes 2 p STARTED  92044  77.3mb 10.0.0.6 estest1
unittypes 4 r STARTED      0    261b 10.0.0.4 estest2
unittypes 4 p STARTED      0    261b 10.0.0.5 estest0
unittypes 3 p STARTED      0    261b 10.0.0.4 estest2
unittypes 3 r STARTED      0    261b 10.0.0.5 estest0
unittypes 0 p STARTED 232252 135.9mb 10.0.0.4 estest2
unittypes 0 r STARTED 232252 135.5mb 10.0.0.6 estest1

What can I do to have stable results for multiple queries?
Currently I'm setting search request preference https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-preference.html but as I'm expected to handle higher loads I'd like to be able to utilize all replicas.