graphing week month day and year, mix result

hello all

i notice that the graph of day (--end now --start end-86400) is accurate but my other graphs are not looks like they are decreasing, say in the day graph u see it gets to 30000 point on the Y-axis but in the week (--end now --start end-604800) its not passing the 25000 point and in
month (--end now --start end-2419200) its not passing the 23000 and
year (--end now --start end-29030400) its not passing the 15000 point

i use the same rrd for all of the graph and the same DS
where is my mistake

thanks alot guys and ladys