Mainly cause they used the Lucavi/Ultimate Summon from FFT(and FFTA Totema) with the correct translations because all 3 take place in Ivalice, which comprises of 7 of the 13 summons,
Lucavi: Belias/Velius-FFT, Adrammelech/Adramelk-FFT/FFTA, Cuchulainn/Queklain-FFT, Zalera/Zalhera-FFT, Hashmal/Hashmalim-FFT, Ultima/Altima-FFT/FFTA
Ultimate Summon: Zodiark/Zodiac-FFT.
Also they used the Totema from FFTA as some summons as well, one of those is already listed previously because it appears in both FFT/FFTA as Lucavi/Totema, which is Adrammelech, the other listed previously is the end boss of FFT, Altima. The Totema that don't appear in FFT are
Totema: Mateus-FFTA/FFII, Famfrit-FFTA/FFIII, Exodus/Exdeath(forever)-FFTA/FFV, which makes the total to 10 of 13 summons.
Next of the lists are the end bosses of FFI-V Espers, 3 listed previously because they appeared as Totema in FFTA, so won't be listing them here(FFII/III/V bosses)
End Bosses: Chaos-FFI, Zeromous-FFIV, which brings the total to 11 of 13 summons, if you have played past FFs and FFT/FFTA the summons are recognizable except one, which brings us to the last, Shemhazai is a totally new creation of SE's for FFXII that was not used in any past FF title, so that brings the total to 13/13.