Actually, it's not like .co.uk in that the .com catching playing field is less regulated and therefore there are deliberate, contractual deals between certain registrars and certain drop-catching companies (and sometimes also internally if a company does both).
In other words, Snapnames will ALWAYS 100% of the time catch names from its registrar drop partners, because they pass the name over via a back door and it never gets into the public domain. This is "legal" and "acceptable" in the .com industry (let's leave a discussion of ethics off the table as it just muddies the water, and stick to the legal aspects)
The only time a partner drop catcher will "fail" to catch a name is if there's zero interest in it, both from any of the customers using them (i.e. nobody booked to catch the name through them) and internally (i.e. nobody at the drop catcher wanted the domains for their internal portfolio).
So that's why it's VITAL to book the name you're after at the right drop partner, otherwise your chance of catching it isn't small - it's zero!
(NOTE: I abandoned the .com drop market because it became so insanely complicated, but there are plenty of people who have put in the time and effort to master it and who are still picking up valuable drops today)