Anyway this is hardly a Costco specific comment - but here is a run of their buy-backs (after issuance/option exercise etc). Negative numbers are net repurchases.
||Cash from stock issuance
The company - as you can see - has bought back a lot of stock. The lack of a buyback in 2013 followed a purchase of a non-controlling stake in Costco Mexico.
But whatever - the company stopped repurchasing stock at the bottom of the market in 2009 - only to start again in earnest as the market and their stock price went up.
This happens in almost only cases - and in this case I do not think the board is mendacious in manipulating their stock. It just happens.
And it even happened with Charlie Munger (who is more than passingly rational) on the board.