If I recall correctly, that can happen due to syncing issues when the site syncs to Steam to update its list. That kind of thing has a tendency to happen during Steam sales, so it may be related to the current timing of things. It could also be that staff was doing something with the listing at the time.

The site determines CV by sending a check against the pricing data of the store page related to the ID of the game in question, and thus should always offer CV so long as the associated store page displays a retail value and isn't glitched. In the case of a game being removed from Steam, staff will [eventually] manually set the CV of the game to its last recorded retail value.

Thus, generally speaking a game should always continue to give you any CV it has been established to have.
The main circumstances which are exceptions to this are:

1. The listing is purchase-disabled but not removed from Steam, or removed but seems likely to be readded in the future, in which case staff will wait to see how things develop [as such games often get purchase re-enabled]. This situation will eventually sort itself out, either by the game returning for purchase, or by staff deciding that it won't be returning, and setting CV manually. Not all staff have the ability to make manual CV edits, and staff regularly lacks for time to handle such matters, so any removed-from-Steam game may temporarily offer 0CV, even if is one that staff already intends to manually edit.
2. The listing is bugged in a way that staff can't work around. One example of this is the listing type changing from a package to a bundle, as the Steam API does not send out data on bundles. In that example, since the listing is still properly listed on the store (and likely available for purchase on third-party sites as keys), staff can't set a manual price, as the price may still fluctuate in the future (and it'd be too much hassle for the staff's limited number of staff to keep track of such for the numerous games affected by such matters).
3. The retail value of the game drops to $0, due to the game going free-to-play.

CV is short for Contribution Value or, put another way, the "experience points" of Steamgifts account levels.