A contrast between sp plot and ggplot with thresholds producing different results

sp_v_ggplot_different.R

library(tidyverse)
library(broom)
library(sp)
library(spdplyr)
library(ggplot2)
library(rgdal)
library(raster)
library(rgeos)
library(lubridate)


#read in spatialPolygonsDataFrame (shapefile)
#from https://www.nceas.ucsb.edu/~byrnes/floating_forests/ff_polys_proj.rds
classifications <- readRDS("./ff_polys_proj.rds")

classifications <- classifications %>% 
  mutate(scene_timestamp = parse_date_time(scene_timestamp, orders="ymdHMS"),
         quarter = quarter(scene_timestamp, with_year=TRUE, fiscal_start=11))

#convert to data frame
classifications_df <- classifications %>% tidy
classifications@data$id <- rownames(classifications@data)
classifications_df <- left_join(classifications_df, classifications@data)

#plot the north coast at a threshold of 6
plot(classifications %>% filter(threshold==6), 
     xlim=c(-124.001673, -122.772273), 
     ylim=c(38.115699,39.176660) )

##ggplot the north coast
ggplot() + 
  geom_polygon(data=classifications_df %>% filter(threshold==6),  
               mapping=aes(x=long, y=lat, group=group,
                           frame = threshold)) +
  theme_bw(base_size=14) +
  xlab("") + ylab("") + 
  xlim(-124.001673, -122.772273) + 
  ylim(38.115699,39.176660)