Different output if SUM and AVG sql function used together and diff. if used individually

SELECT sd.site_id, sd.site_name, sd.site_address, SUM(eb_time) as eb_time, avg(av.dc_volt) as dc_volt FROM site_details sd INNER JOIN alarm_time at ON at.site_id = sd.site_id INNER JOIN avg_values av ON av.site_id = sd.site_id GROUP BY sd.site_id, sd.site_name, sd.site_address;
For this the output is
S001 10406 36.085000 S002 3376 29.010000 S003 42 29.000000
SELECT sd.site_id SUM(eb_time) as eb_time
avg(av.dc_volt) as dc_volt //If i remove this and 
FROM site_details sd INNER JOIN alarm_time at ON at.site_id = sd.site_id INNER JOIN avg_values av ON av.site_id = sd.site_id// this the output is asshown below GROUP BY sd.site_id, sd.site_name, sd.site_address;
S001 10406 S002 3376 S003 42
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