Bidding above the top CPM means that the campaign will be placed in the first position within the auction.
Campaigns in the first position in the auction have the highest priority to be displayed.
Matching the top CPM means that the campaign will share the first position in the auction. It is possible to share a slot with one or more campaigns. In this case impressions will be shared with all these campaigns.
For example, if the top bid is currently 0.2, bidding 0.21 will achieve first position for the campaign, but it will likely impressions it with campaigns bidding 0.2.
The platform has a complex algorithm, with many variables involved and it is not possible to foresee what amount would trigger the creation of a new slot.
In this specific example, we would suggest to bid more than 0.25 in order to trigger the creation of a new top position without sharing with other campaigns.
Please be aware that top CPM of a campaign should not be considered as a static value: changes of the top bid will always be visible also to other advertisers, who might adjust their bids according to the new value.