That list only shows the most recent ones.
To get all the logins in the past month you will have to use a custom report.
This report lists all logins during the past 31 days:
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tblCPlogoninfo.Username,
tblCPlogoninfo.Domain As Userdomain,
tblCPlogoninfo.logontime,
tsysOS.Image As icon,
tsysOS.OSname
From tblCPlogoninfo
Inner Join tblAssets On tblAssets.AssetID = tblCPlogoninfo.AssetID
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Where tblCPlogoninfo.logontime > GetDate() - 31
Order By tblAssets.AssetName,
tblCPlogoninfo.logontime Desc