New Gmail account - never shared yet full of spam?

[#1]

A few months ago I set up a Gmail account, I can't remember why but I certainly have not used it for anything. I logged into it today to test sending an email since somebody is having problems receiving from my Yahoo account and noticed that the spam folder is full of spam. I chose forenamesurnameuk@gmail.com - is it simply an easily-guessable address (my Yahoo account is forenamesurnameuk@yahoo.co.uk but doesn't receive any spam!) or is there somewhere in the million Google settings which has shared it. The account has not been used for any Google service (Google Checkout etc.)

[#2]

Have you used it for anything at all to purchase a service?
I have two accounts for Gmail for various reasons.
My main one and newer one is now 87% full and gets around 20 spam messages a week, around 20% of these are not detected and come to my inbox. My original one is only 4% full, received in excess of 450 spam messages a week and very little to the inbox.
Thankfully, they support the format of address email+subbox@gmail.com, and I can put anything I want to the subbox portion. I make this unique for any singup, so I can see  who is selling my details.
I already know it is insurance brokers and prior financial enquiries that has resulted in my details being sold, but which one I will never prove.

[#3]

I got spam when I set up a new gmail account before I had given the address to anyone - it was by a dictionary attack.

[#4]

Spammers know that people like to use their name as the first part of their email, even if they have several domains.
Someone had your yahoo address and used to guess your gmail account - not hard.

[#5]

Dictionary attacks I think. I have several Gmail accounts - the ones where I have simply used my name or some combination of names (e.g. fredandmabel) get spam. The ones where I have not used a name but instead a combination of letters and numbers never get spam.