Variation within a chromatography with the same sample

What are some possible explanations to why the exact same sample in SEC HPLC differ in their peak heights, although it's the same chromatographic profile?


The sample contains cell culture medium (undiluted) into a 10mM phosphate NaCl buffer. Is it possible that the sample is overloaded, might this be the cause for such different results? 


After each sample there's a 1x column bed volume (24ml) wash with the phosphate buffer.


Any theories based on experience or otherwise?


Edit:


**Sample 1:** 


Peak 1: 166292μV, 


Peak 2: 199835μV,


Peak 3: 168024μV,


Peak 4: 78483μV


**Sample 2:** 


Peak 1: 96936μV, 


Peak 2: 142119μV,


Peak 3: 148060μV,


Peak 4: 195584μV


**Sample 3:** 


Peak 1: 56769μV, 


Peak 2: 88114μV,


Peak 3: 82351μV,


Peak 4: 176517μV


Chromatogam looks something like this (Peak count from left to right)


[![enter image description here](https://i.stack.imgur.com/BQVQL.jpg)](https://i.stack.imgur.com/BQVQL.jpg)


