rbind and spRbind

I'm a bit confused with the following:

I have a SPDF object ika1v4

dim(ika1v4geo at data)

[1] 693  23

dim(ika1v4geo)

[1] 693  23

Then I do an rbind:

a <- rbind(ika1v4geo,ika1v4geo)
dim(a)

[1] 1386   23

dim(a at data)

[1] 1386   23

But spRbind introduces 2 variables:

a2 <- spRbind(ika1v4geo,ika1v4geo)
dim(a2)

[1] 1386   25

dim(a2 at data)

[1] 1386   25

names(a at data)

 [1] "IKAPUNTO" "IKAID"    "CN"       "dia"      "hora_obs" "GPSID"
 [7] "GPS"      "N_PUNTO"  "GUIA"     "MONITOR"  "COD_SP_C" "SP_C"
[13] "COD_OBS"  "OBS"      "N"        "VULN"     "VULNT"    "CL"
[19] "A"        "H"        "D"        "S"        "V"

names(a2 at data)

 [1] "IKAPUNTO" "IKAID"    "CN"       "dia"      "hora_obs" "GPSID"
 [7] "GPS"      "UTMX"     "UTMY"     "N_PUNTO"  "GUIA"     "MONITOR"
[13] "COD_SP_C" "SP_C"     "COD_OBS"  "OBS"      "N"        "VULN"
[19] "VULNT"    "CL"       "A"        "H"        "D"        "S"
[25] "V"

Why? Which are the differences between rbind and spRbind?

Thanks!

Agus