Block root in checkpoint is not the block inside the first slot of given epoch in checkpoint ?

[mitmotw]

Hi all,

prysm/beacon-chain/blockchain/process_attestation_helpers.go

Line 27 in 09d761e

if err == nil && slots.ToEpoch(targetSlot)+1 >= headEpoch {

As the title says,
I thought the block root in checkpoint is the block inside the first slot of given epoch in checkpoint
which means that
slots.ToEpoch(targetSlot) == c.epoch
in above getAttPreState function.

When I checked the blocks in explorer, that seemed right.
Could you give me some hint to understand this?
In which case, it's not guaranteed that the epoch of 'given block root' is not 'the given epoch'?