postgres vacuum memory limits

Hello, when maintenance_work_mem and autovacuum_work_mem are set, my
understanding is that the vacuum and autovacuum sessions should be limited
to use the memory limits set by these parameters. But I am seeing more
memory being used than these limits by autovacuum sessions, any reason why
this would happen?

Please see below examples, where maintenance_work_mem is set to 20mb and
shared_buffers is 128mb. When I see the memory for this session in top, it
shows 162mb. But when default_statistics_target is increased to 3000, the
session usage is 463mb, which is way more than 20mb maintenance_work_mem
and 128mb shared_buffer. Shouldn't the process memory be capped to 20+128mb?

postgres=# show maintenance_work_mem ;
maintenance_work_mem ----------------------
20MB
(1 row)
postgres=# vacuum analyze mdm_context;
VACUUM
postgres=# show shared_buffers;
shared_buffers ----------------
128MB
(1 row)

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
COMMAND
62246 postgres 20 0 422892 165996 139068 R 57.1 15.7 25:06.34
postgres: postgres postgres [local] VACUUM

postgres=# show default_statistics_target;
default_statistics_target ---------------------------
100
(1 row)
postgres=# set default_statistics_target=3000;SET
postgres=# vacuum analyze mdm_context;
VACUUM

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
COMMAND
62246 postgres 20 0 876132 474384 2976 R 62.9 47.6 25:11.41
postgres: postgres postgres [local] VACUUM