I spend more than 10.00 here and there. lol... I have bought them when they are not on sale and when they are on sale.
I really do believe these go out random. There are lot of players that do not buy gems until they go on sale so it does not really benefit TL to hand pick players to send these sales to. Bottom line, people are either going to buy or not buy gems... now some people that hardly ever buy, maybe more tempted to buy a few when there is a sale.
I have iphone(main game),, then I also have ipad 3, Itouch 4 and kindle fire that I have games downloaded on but do not play, and I have not seen pattern on the sales. I get almost the same amount of sales on my active device as the non-active devices just at different times.

And yes, TL is a business and of course depends on players that buy gems to pay the bills and keep the games running, and also make it possible for other players that never buy gems to play for free.

Same thing happens in the real world, there is on big department store that when they sent out flyers to credit card holders, they will have a peel off where you can get random sales. 15% 20% and 30%. My husband gets one and I get one in the mail. sometimes I get the 30% off and sometimes he does, it is always random. My credit card is the one used, he never uses his(unless for my present. lol).