counting returns different results

[Sean Huo]

These two queries returns vastly different results:

query1:
select count(1) from t where date > 'certain day' and device ='A';

query2:

select device, count(1) from t where date >'certain day' and device ='A'
group by device;

assuming table t has the  composite primary key of
date timestamp, and device varchar.

What gives?