telethon GetParticipantsRequest: why returning not all members

Question

telethon GetParticipantsRequest: why returning not all members ? For example, my groups has 3377 members, but GetParticipantsRequest return 3356 members.

while True:
    participants = await client(GetParticipantsRequest(
        my_channel, ChannelParticipantsSearch(''), offset, limit,
        hash=0
    ))
    if not participants.users:
        break
    all_participants.extend(participants.users)
    offset += len(participants.users)

If print participants:

if not participants.users:
    print(participants)
    break

I get:

ChannelParticipants(count=3389, participants=[], chats=[], users=[])

count=3389 and it's not equals 3377 (count of members in my group).
Why???

Plese help me with it.