Security prices on most exchanges move in units of a minimum size.
For example, London listed shares will usually have prices that are multiples of 0.25p. So a share can have a price of 10.25p or 10.50p, but not, for example 10.3p.
This applies to bid, offer and last traded prices. Mid-prices and other averages, such as VWAP prices are not restricted by the tick.