strange query performance

[Eugene G]

1. average time 1.3 s

SELECT  
sql_no_cache
ClientSeriousness.*
FROM Clients INNER JOIN ClientSeriousness ON Clients.SeriousnessID = ClientSeriousness.SeriousnessID
WHERE Clients.ProjectID=1390;

describe gives the following info

id,select_type,table,type,possible_keys,key,key_len,ref,rows,Extra
1,SIMPLE,ClientSeriousness,ALL,PRIMARY,NULL,NULL,NULL,4,
1,SIMPLE,Clients,ref,ProjectID,SeriousnessID,SeriousnessID,5,ClientSeriousness.SeriousnessID,5,Using where

2. average time 0.06 s

SELECT sql_no_cache count(*) from Clients where ProjectID = 1390

3. average time 0.03 s

SELECT sql_no_cache count(*) from ClientSeriousness

How could it be ?