Too many tombstones using TTL

[Python_Max]

Hello, C* users and experts.

I have (one more) question about tombstones.

Consider the following example:
cqlsh> create keyspace test_ttl with replication = {'class':
'SimpleStrategy', 'replication_factor': '1'}; use test_ttl;
cqlsh> create table items(a text, b text, c1 text, c2 text, c3 text,
primary key (a, b));
cqlsh> insert into items(a,b,c1,c2,c3) values('AAA', 'BBB', 'C111', 'C222',
'C333') using ttl 60;
bash$ nodetool flush
bash$ sleep 60
bash$ nodetool compact test_ttl items
bash$ sstabledump mc-2-big-Data.db

[
  {
    "partition" : {
      "key" : [ "AAA" ],
      "position" : 0
    },
    "rows" : [
      {
        "type" : "row",
        "position" : 58,
        "clustering" : [ "BBB" ],
        "liveness_info" : { "tstamp" : "2018-01-10T13:29:25.777Z", "ttl" :
60, "expires_at" : "2018-01-10T13:30:25Z", "expired" : true },
        "cells" : [
          { "name" : "c1", "deletion_info" : { "local_delete_time" :
"2018-01-10T13:29:25Z" }
          },
          { "name" : "c2", "deletion_info" : { "local_delete_time" :
"2018-01-10T13:29:25Z" }
          },
          { "name" : "c3", "deletion_info" : { "local_delete_time" :
"2018-01-10T13:29:25Z" }
          }
        ]
      }
    ]
  }
]

The question is why Cassandra creates a tombstone for every column instead
of single tombstone per row?

In production environment I have a table with ~30 columns and It gives me a
warning for 30k tombstones and 300 live rows. It is 30 times more then it
could be.
Can this behavior be tuned in some way?

Thanks.

-- 
Best regards,
Python_Max.