hCG doubles every 48 hours. You can have a level of 1 - 5 in a non pregnant state. 

Implantation could be later than you think. 

If a woman implants at 8dpo, with a 'non pregnant' hCG level of 1 [Which is totally normal] it would rise as follows

8dpo - 1miu hcg 10dpo - 2miu 12dpo - 4miu 14dpo [Day AF is typically due] - 8miu 16dpo - 16miu 

So this lady wouldn't finnd out until 15dpo on a very senstive 10miu pregnancy test, superdrug own or possibly asda own. Obviously if she implants LATER than 8dpo [which is normal] she will find out even later.

Now, Take a lady who implants at 8dpo, But has a'Non pregnant' hCG level of 5. Hers looks like this;

8dpo - 5miu 10dpo- 10miu - Could find out here on superdrug own, internet cheapie 12dpo - 20miu - Could find out here with freedom cheapie strip/most poundland tests and asda own] 14dpo - 40miu [day period is typically due] Most tests would pick it up here

So, even though implantation is on the same day for both ladies, one can find out MUCH sooner. Now, If this lady were to implant at 6dpo she could potentially find out at just 8dpo. But if the first lady were to implant at 12dpo, She would find out even later, Even if they have the same cycle and OV days!